osdWindow: Show on all monitors

We had one osdWindow that we displayed either on the primary monitor or on
whatever monitor index got passed over dbus.

Change that to show the osd on all monitors when no explicit monitor is
requested. A monitor should be requested in cases like display brightness where it makes sense to only show the osd on the affected monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=722684