AppDisplay: fake a release event to dnd when opening the menu

PopupMenuManager eats the next release event, which would otherwise close
the menu, so we need to tell dnd that we're handling it, and no drag should
be started, so it can ungrab the pointer and restore state.

https://bugzilla.gnome.org/show_bug.cgi?id=694276

js/ui/appDisplay.js

@@ -790,6 +790,7 @@ const AppIcon = new Lang.Class({
     popupMenu: function() {
         this._removeMenuTimeout();
         this.actor.fake_release();
+        this._draggable.fakeRelease();
 
         if (!this._menu) {
             this._menu = new AppIconMenu(this);