In case the `wlr_input_device` is not owned by a specialized input device, the
function will finish the wlr_input_device and call it's implementation destroy
function if an implementation has been supplied, or simply free it.
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
