Add wl_client late-destroy listener
The wl_client
destroy listener is called before any of the client's resources are destroyed. Some users may want a destroy handler for after the resources are destroyed, rather than having to disperse their cleanup everywhere. Since moving the destroy listener would be an effective ABI break, add a new 'late-destroy' listener which will be called after all the client's resources have been destroyed.
Fixes: #207 (closed)