dash: Don't allow to remove running apps from favorites

Running apps are always kept in the dash, so removing them from
favorites just moves them to the end of the favorites list. This
behavior is not immediately obvious, so only show the remove target
when dragging a favorites application that is not currently running.

https://bugzilla.gnome.org/show_bug.cgi?id=644853

js/ui/dash.js

@@ -387,6 +387,7 @@ const Dash = new Lang.Class({
         let srcIsFavorite = (id in favorites);
 
         if (srcIsFavorite &&
+            app.get_state() != Shell.AppState.RUNNING &&
             dragEvent.source.actor &&
             this.actor.contains (dragEvent.source.actor) &&
             this._favRemoveTarget == null) {