ctrl-alt-tab: use better icons

The top panel and message tray icons were by default a gnome foot and
are replaced by better ones. The applications icon is now using the
symbolic apps icon of the dash, and the windows icon is also improved.

https://bugzilla.gnome.org/show_bug.cgi?id=641303

js/ui/messageTray.js

@@ -1565,7 +1565,7 @@ const MessageTray = new Lang.Class({
         if (Main.sessionMode.isLocked || Main.sessionMode.isGreeter)
             Main.ctrlAltTabManager.removeGroup(this._summary);
         else
-            Main.ctrlAltTabManager.addGroup(this._summary, _("Message Tray"), 'start-here-symbolic',
+            Main.ctrlAltTabManager.addGroup(this._summary, _("Message Tray"), 'user-available-symbolic',
                                             { focusCallback: Lang.bind(this, this.toggleAndNavigate),
                                               sortGroup: CtrlAltTab.SortGroup.BOTTOM });
         this._updateState();