backend/wayland: give wlr_pointer ownership to wlr_wl_seat

All the code logic related to the pointer has been moved to its own file.

The seat is responsible for the lifetime of its wlr_wl_pointer(s), and assigning
them to the relevant wlr_wl_output. The wlr_wl_pointer becomes a simple helper
to manager the wlr_pointer associated to the seat's wl_pointer and its lifetime.

backend/wayland/meson.build

@@ -8,6 +8,7 @@ wlr_files += files(
 	'backend.c',
 	'output.c',
 	'seat.c',
+	'pointer.c',
 	'tablet_v2.c',
 )