GtkApplication: fixes for NULL session bus

GtkApplication: fixes for NULL session bus

We currently have a couple of cases where GtkApplication assumes that
the session bus will be non-NULL causing critical error output or (in
the case of trying to publish menus) an infinite loop.

Three fixes:

 - if the session bus is NULL due to not having registered the
   GtkApplication yet then give a g_critical on the entry point to the
   menu setters instead of going into an infinite loop.  Document this.

 - check for NULL session bus even when calling the menu setters at the
   right time in order to prevent the infinite loop for
   non-programer-error cases (ie: because we had trouble connecting to
   the session bus)

 - check for NULL session bus when publishing the X11 properties on the
   GtkApplicationWindow and skip publishing them if we're not on the bus

https://bugzilla.gnome.org/show_bug.cgi?id=671249