When determining if there is a non-GTK modal window running, we can't just
assume that any NSWindow that is not associated with a GtkWindow is a
modal window.  On Mavericks when we go into fullscreen mode we get an
NSStatusBarWindow in the list which is not associated with a GtkWindow, so
we must now check for that.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=14469