ShellAppSystem: Support loading a .desktop file directly

Previously, ShellAppSystem only loaded (and cached) the set of
.desktop files from applications.menu and settings.menu, using
the gnome-menus library.  The ShellAppInfo structure was
a "hidden typedef" for GMenuTreeEntry.

But we need to support loading an arbitrary .desktop file.  Thus,
refactor the ShellAppInfo into a real struct, with a refcount,
and allow it to point to either a GMenuTreeEntry or a GKeyFile.

Also, in the case where we fail to lookup an icon for an
application, ensure we return a 0 opacity texture.

js/ui/widget.js

@@ -318,7 +318,7 @@ AppsWidget.prototype = {
         let appSystem = Shell.AppSystem.get_default();
         let apps = appSystem.get_favorites();
         for (let i = 0; i < apps.length; i++) {
-            let app = appSystem.lookup_app(apps[i]);
+            let app = appSystem.lookup_cached_app(apps[i]);
             if (!app)
                 continue;
             this.addItem(new AppsWidgetInfo(app));