Make gtk_init_check work as expected

Make gtk_init_check work as expected

When no GDK backend can be initialized (either because
GDK_BACKEND has been set to the wrong value, or the backends
are simply not included), the expectation is that gtk_init_check
should return FALSE, not error out.
This commit makes it so, by using gdk_display_manager_peek
instead of gdk_display_manager_get in code paths that are used
during initialization.