workspace: Show window chrome on long-press

Pointer hover and keyboard focus aren't available on touch devices,
so add long-press as alternative to reveal preview title and close
button.

https://bugzilla.gnome.org/show_bug.cgi?id=783953

js/ui/workspace.js

@@ -380,6 +380,8 @@ var WindowClone = new Lang.Class({
                     action.release();
                     this._draggable.startDrag(x, y, global.get_current_time(), this._dragTouchSequence);
                 }));
+        } else {
+            this.emit('show-chrome');
         }
         return true;
     },