Consistently exit the overview when launching external applications

We were doing this in one or two places, but not most of them.

https://bugzilla.gnome.org/show_bug.cgi?id=653095

js/ui/dateMenu.js

@@ -198,6 +198,7 @@ DateMenuButton.prototype = {
 
     _onPreferencesActivate: function() {
         this.menu.close();
+        Main.overview.hide();
         let app = Shell.AppSystem.get_default().get_app('gnome-datetime-panel.desktop');
         app.activate(-1);
     },