switcherPopup: Cancel the OSD popup before showing

It is irritating to show two different system popups at the same
time, so let switcher popups like alt-tab cancel out the OSD.

https://bugzilla.gnome.org/show_bug.cgi?id=613543

js/ui/switcherPopup.js

@@ -160,6 +160,7 @@ const SwitcherPopup = new Lang.Class({
         // disturbed by the popup briefly flashing.
         this._initialDelayTimeoutId = Mainloop.timeout_add(POPUP_DELAY_TIMEOUT,
                                                            Lang.bind(this, function () {
+                                                               Main.osdWindow.cancel();
                                                                this.actor.opacity = 255;
                                                                this._initialDelayTimeoutId = 0;
                                                            }));