Configure panel thickness, length, and anchor per-monitor

Settings.ui:
- Move thickness, length, and anchor from Style tab to Position tab.
Group together with position in the same frame.

schema:
- Remove unpublished panel-length and panel-anchor settings, replacing
them with panel-lengths and panel-anchors JSON objects (like
panel-positions).
- Remove unpublished anchor enum, since panel-anchors is being managed
by the extension in JSON, not typed by the schema.
- Deprecate panel-size in favour of new panel-sizes JSON, storing
per-monitor panel sizes.
- Mention that panel-position is deprecated.

Introduce panelSettings.js:
- Functions to fetch or set panel settings that are stored as JSON.
Grown from now-removed getSettingsPositions() in panelPositions.js.

prefs.js:
- Group together the different UI widget label and value refreshing
into method _updateWidgetSettingsForMonitor().
- Change multi-panel behaviour of _setAnchorLabels(). Previously, all
panels shared the same anchor setting, and so setAnchorLabels
considered all monitors. Now, panels are configured either
independently, or sometimes all together; set labels according to
orientation(s) of panel(s) being configured.
- Omitting preventTop handling in refactored _setPanelPosition()
method. Previously, it was written to set the first monitor's panel to
Pos.BOTTOM if the user clicked the Top position radio button, if
stockgs-keep-top-panel was also set. But the user can't activate the
Top button anyway if stockgs is set (likely implemented later).

panelManager.js:
- Removing panelPositions, as it is not needed any more.

panelSettings.js

@@ -0,0 +1,112 @@
+/*
+ * This file is part of the Dash-To-Panel extension for Gnome 3
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+const Me = imports.misc.extensionUtils.getCurrentExtension();
+const Pos = Me.imports.panelPositions;
+
+/** Return object representing a settings value that is stored as JSON. */
+function getSettingsJson(settings, setting) {
+    try {
+        return JSON.parse(settings.get_string(setting));
+    } catch(e) {
+        log('Error parsing positions: ' + e.message);
+    }
+}
+/** Write value object as JSON to setting in settings. */
+function setSettingsJson(settings, setting, value) {
+    try {
+        const json = JSON.stringify(value);
+        settings.set_string(setting, json);
+    } catch(e) {
+        log('Error serializing setting: ' + e.message);
+    }
+}
+
+/** Returns size of panel on a specific monitor, in pixels. */
+function getPanelSize(settings, monitorIndex) {
+    const sizes = getSettingsJson(settings, 'panel-sizes');
+    // Pull in deprecated setting if panel-sizes does not have setting for monitor.
+    const fallbackSize = settings.get_int('panel-size');
+    const theDefault = 48;
+    return sizes[monitorIndex] || fallbackSize || theDefault;
+}
+
+function setPanelSize(settings, monitorIndex, value) {
+    if (!(Number.isInteger(value) && value <= 128 && value >= 16)) {
+        log(`Not setting invalid panel size: ${value}`);
+        return;
+    }
+    let sizes = getSettingsJson(settings, 'panel-sizes');
+    sizes[monitorIndex] = value;
+    setSettingsJson(settings, 'panel-sizes', sizes);
+}
+
+/**
+ * Returns length of panel on a specific monitor, as a whole number percent,
+ * from settings. e.g. 100
+ */
+function getPanelLength(settings, monitorIndex) {
+    const lengths = getSettingsJson(settings, 'panel-lengths');
+    const theDefault = 100;
+    return lengths[monitorIndex] || theDefault;
+}
+
+function setPanelLength(settings, monitorIndex, value) {
+    if (!(Number.isInteger(value) && value <= 100 && value >= 0)) {
+        log(`Not setting invalid panel length: ${value}`);
+        return;
+    }
+    let lengths = getSettingsJson(settings, 'panel-lengths');
+    lengths[monitorIndex] = value;
+    setSettingsJson(settings, 'panel-lengths', lengths);
+}
+
+/** Returns position of panel on a specific monitor. */
+function getPanelPosition(settings, monitorIndex) {
+    const positions = getSettingsJson(settings, 'panel-positions');
+    const fallbackPosition = settings.get_string('panel-position');
+    const theDefault = Pos.BOTTOM;
+    return positions[monitorIndex] || fallbackPosition || theDefault;
+}
+
+function setPanelPosition(settings, monitorIndex, value) {
+    if (!(value === Pos.TOP || value === Pos.BOTTOM || value === Pos.LEFT
+        || value === Pos.RIGHT)) {
+        log(`Not setting invalid panel position: ${value}`);
+        return;
+    }
+    const positions = getSettingsJson(settings, 'panel-positions');
+    positions[monitorIndex] = value;
+    setSettingsJson(settings, 'panel-positions', positions);
+}
+
+/** Returns anchor location of panel on a specific monitor. */
+function getPanelAnchor(settings, monitorIndex) {
+    const anchors = getSettingsJson(settings, 'panel-anchors');
+    const theDefault = Pos.MIDDLE;
+    return anchors[monitorIndex] || theDefault;
+}
+
+function setPanelAnchor(settings, monitorIndex, value) {
+    if (!(value === Pos.START || value === Pos.MIDDLE || value === Pos.END)) {
+        log(`Not setting invalid panel anchor: ${value}`);
+        return;
+    }
+    const anchors = getSettingsJson(settings, 'panel-anchors');
+    anchors[monitorIndex] = value;
+    setSettingsJson(settings, 'panel-anchors', anchors);
+}