wayland: Add MetaWaylandTablet

This (very basic at the moment) struct keeps server-side information
for wl_tablet resources.

src/wayland/meta-wayland-types.h

@@ -37,6 +37,9 @@ typedef struct _MetaWaylandDragDestFuncs MetaWaylandDragDestFuncs;
 typedef struct _MetaWaylandDataOffer MetaWaylandDataOffer;
 typedef struct _MetaWaylandDataDevice MetaWaylandDataDevice;
 
+typedef struct _MetaWaylandTabletSeat MetaWaylandTabletSeat;
+typedef struct _MetaWaylandTablet MetaWaylandTablet;
+
 typedef struct _MetaWaylandBuffer MetaWaylandBuffer;
 typedef struct _MetaWaylandRegion MetaWaylandRegion;