When running with a Wayland compositor which doesn't support the
xdg_shell interface, gtk+ will segfault while trying to access the
corresponding wl proxy.
Check for xdg_shell support and do not use Wayland if not present, so
that it can fallback to X11, hoping that Xwayland is usable.
Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=762258