wayland: Process the keymap that is sent over from the compositor

Load the keymap from the file descriptor that the compositor has sent us and
then save that into our internal object for future use.

gdk/wayland/gdkdisplay-wayland.h

@@ -87,6 +87,8 @@ struct _GdkWaylandDisplay
 
   GSource *event_source;
 
+  struct xkb_context *xkb_context;
+
 #ifdef GDK_WAYLAND_USE_EGL
   EGLDisplay egl_display;
   EGLContext egl_context;