GtkApplication: Try to cope with ssh situations better

GtkApplication: Try to cope with ssh situations better

Override the gtk-shell-shows-app-menu and gtk-shell-shows-menubar
settings to FALSE, if we can detect that we are not on the same
session bus as the xsettings provider that we got these settings
from.

We determine this by comparing the bus ID of 'our' session
bus with the one reported in the Gtk/SessionBusId xsetting.
If they are different, then it very likely that we are in an ssh
situation where we see the forwarded X display, but not the
session bus. The D-Bus based menu exporting will not work
in this situation.

https://bugzilla.gnome.org/show_bug.cgi?id=671802