wayland: Add MetaWaylandTabletSeat

This object groups all the tablets/tools pertaining to a given seat.

src/wayland/meta-wayland-types.h

@@ -37,6 +37,7 @@ typedef struct _MetaWaylandDragDestFuncs MetaWaylandDragDestFuncs;
 typedef struct _MetaWaylandDataOffer MetaWaylandDataOffer;
 typedef struct _MetaWaylandDataDevice MetaWaylandDataDevice;
 
+typedef struct _MetaWaylandTabletManager MetaWaylandTabletManager;
 typedef struct _MetaWaylandTabletSeat MetaWaylandTabletSeat;
 typedef struct _MetaWaylandTabletTool MetaWaylandTabletTool;
 typedef struct _MetaWaylandTablet MetaWaylandTablet;