dash: simplify app retrieval from AppWellIcon

This has also the benefit of getting the application even if it can not
be retrieved through AppSystem, which can happen if the runtime WMClass
does not match the one of the desktop file.

This especially looked wrong with the following commits related to the
bug.

https://bugzilla.gnome.org/show_bug.cgi?id=657315

js/ui/dash.js

@@ -23,8 +23,7 @@ const DASH_ITEM_HOVER_TIMEOUT = 300;
 
 function getAppFromSource(source) {
     if (source instanceof AppDisplay.AppWellIcon) {
-        let appSystem = Shell.AppSystem.get_default();
-        return appSystem.lookup_app(source.getId());
+        return source.app;
     } else if (source.metaWindow) {
         let tracker = Shell.WindowTracker.get_default();
         return tracker.get_window_app(source.metaWindow);