foreign-toplevel-management: Allow to emit toplevels responsiveness state

AFAIK we currently don't have a way for the compositor to inform interested clients about toplevels not answering to ping events. Adding that functionality would allow desktop shells to inform the user about the state of the app (and e.g. allow to force quit it).

I'm not 100% happy about the responsiveness terminology but pingable seemed to restrictive as compositors might use different methods to determine that a toplevel is still usable by the user.