wayland: split headers and distribute structure definitions

Instead of having all structures in one huge headers, move them
in the appropriate place, and create one header for surface state.

https://bugzilla.gnome.org/show_bug.cgi?id=707128

src/wayland/meta-wayland-data-device.c

@@ -33,6 +33,7 @@
 #include "meta-wayland-data-device.h"
 #include "meta-wayland-seat.h"
 #include "meta-wayland-pointer.h"
+#include "meta-wayland-private.h"
 
 static void
 data_offer_accept (struct wl_client *client,