wayland: Implement wp_tablet_pad_strip

This represents pad sliders.

src/wayland/meta-wayland-types.h

@@ -41,6 +41,7 @@ typedef struct _MetaWaylandTabletManager MetaWaylandTabletManager;
 typedef struct _MetaWaylandTabletSeat MetaWaylandTabletSeat;
 typedef struct _MetaWaylandTabletTool MetaWaylandTabletTool;
 typedef struct _MetaWaylandTablet MetaWaylandTablet;
+typedef struct _MetaWaylandTabletPadStrip MetaWaylandTabletPadStrip;
 typedef struct _MetaWaylandTabletPadRing MetaWaylandTabletPadRing;
 
 typedef struct _MetaWaylandBuffer MetaWaylandBuffer;