Alt-F2: Avoid running programs multiple times

Returning true from the event handler seems to avoid getting
duplicate return events; these duplicate events likely are
getting generated by IBus.

https://bugzilla.gnome.org/show_bug.cgi?id=644509

js/ui/runDialog.js

@@ -251,6 +251,7 @@ __proto__: ModalDialog.ModalDialog.prototype,
                     if (!this.pushModal())
                         this.close();
                 }
+                return true;
             }
             if (symbol == Clutter.Escape) {
                 this.close();