wlr_tablet_pad owns its wlr_input_device. It will be initialized when the tablet pad is initialized, and finished when the tablet pad is destroyed.
This introduces -DWLR_USE_UNSTABLE and adds information regarding the stability status to all headers. I started with a conservative set of headers to mark as stable: - types/wlr_matrix.h - util/edges.h - util/log.h - util/region.h - xcursor.h
TODO: Update the code accordingly and move other types into the same paradigm
