[api,doc] Add stylesheed for doxygen

As suggested by @ctrlaltca:matrix.org the project logo now uses a
CSS to properly align its size.

docs/Doxyfile

@@ -1280,7 +1280,7 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  =
+HTML_EXTRA_STYLESHEET  = FreeRDP.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note

docs/FreeRDP.css

@@ -0,0 +1,5 @@
+#projectlogo img {
+  border: 0px none;
+  height: 48px;
+  padding: 5px;
+}