Using `dlsym` for `gdk_wayland_display_get_type` is a cleaner solution
to bug 1696319, allowing running with a GTK that lacks the Wayland
backend.
Also adds a symmetric implementation for `gdk_x11_display_get_type`,
which should help running without X11.
Differential Revision: https://phabricator.services.mozilla.com/D107406