Replace Dash application display with AppWell

The new class AppWell implements the application favorite well
in the Dash component.  The previous AppDisplay remains for use
in the More... mode now.

Delete DEFAULT_APPLICATIONS; this is now in GConf.

Rename getMostUsedApps to getTopApps since we now have the
idea of explicit favorites.

Delete some GenericDisplay-related calls from overlay related
to the seletion - we'll reimplement keyboard nav in a more
coherent way later.

js/ui/widget.js

@@ -284,7 +284,7 @@ AppsWidget.prototype = {
         this.actor = new Big.Box({ spacing: 2 });
         this.collapsedActor = new Big.Box({ spacing: 2});
 
-        let apps = AppInfo.getMostUsedApps(5);
+        let apps = AppInfo.getTopApps(5);
         for (let i = 0; i < apps.length; i++)
             this.addItem(apps[i]);
     }