AppMenuButton: only show the button if the target app is on the current ws

When the last window on a workspace is closed the focus goes to some other
window in another workspace which would cause us to show the AppMenuButton for
an application that isn't visible on the current empty workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=643867

js/ui/panel.js

@@ -497,6 +497,9 @@ const AppMenuButton = new Lang.Class({
             return;
         }
 
+        if (!targetApp.is_on_workspace(workspace))
+            return;
+
         if (!this._targetIsCurrent) {
             this.actor.reactive = true;
             this._targetIsCurrent = true;