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- Remove unpublished panel-length and panel-anchor settings, replacing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- Remove unpublished anchor enum, since panel-anchors is being managed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.

windowPreview.js

@@ -144,7 +144,7 @@ var PreviewMenu = Utils.defineClass({
             [
                 Me.settings,
                 [
-                    'changed::panel-size',
+                    'changed::panel-sizes',
                     'changed::window-preview-size',
                     'changed::window-preview-padding',
                     'changed::window-preview-show-title'