Add application not responding detection.
This is done via a new per-connection “ping” event at the same level as lifecycle events or display configuration events. Clients are expected to reply to this event by calling mir_connection_pong().
By default pings at 1Hz; applications which fail to respond for a full ping cycle are considered unresponsive.
Approved by PS Jenkins bot, Cemil Azizoglu, Kevin DuBois, Robert Carr.