panel: Set up 'open-state-changed' handler on menu changes

Commit 08690d658f generalized the banner-blocking behavior of the
dateMenu to all menus that would obscure the banner. However setting
up the 'open-state-changed' handler only when an indicator is added
does not work for indicators that change their entire menu (like the
app menu) - we currently end up with menus with no connected signal
handler, and throw an error when trying to disconnect an invalid
signal ID.
To address this, add a new PanelButton::menu-set signal and use that
to set up the 'open-state-changed' handler.

https://bugzilla.gnome.org/show_bug.cgi?id=745910

js/ui/panelMenu.js

@@ -128,6 +128,7 @@ const Button = new Lang.Class({
             Main.uiGroup.add_actor(this.menu.actor);
             this.menu.actor.hide();
         }
+        this.emit('menu-set');
     },
 
     _onEvent: function(actor, event) {