More for applications now displays a category list

Rework the previously extant Application category code to display
in the expanded list.  Add a "Frequent" category which corresponds
to the most_used_apps, and selected by default.

Instead of adding the background and shadow as expanded items to
the results/details panes as fixed, we slave the background/shadow
sizes to the results using notify::allocation.

Also clean up the code for sizing the details pane, using a common
function which adjusts its x position in one place.

js/ui/genericDisplay.js

@@ -355,7 +355,7 @@ GenericDisplay.prototype = {
         this._selectedIndex = -1;
         // These two are public - .actor is the normal "actor subclass" property,
         // but we also expose a .displayControl actor which is separate.
-        // See also getSideArea.
+        // See also getNavigationArea.
         this.actor = this._list;
         this.displayControl = new Big.Box({ background_color: ITEM_DISPLAY_BACKGROUND_COLOR,
                                             spacing: 12,
@@ -463,7 +463,7 @@ GenericDisplay.prototype = {
 
     // Returns an actor which acts as a sidebar; this is used for
     // the applications category view
-    getSideArea: function () {
+    getNavigationArea: function () {
         return null;
     },