Games do not stop when Mir window closed

[wmww]

To reproduce:

• Open Mircade on X (haven't tested any other way of running it, tried both current stable and the version I built from the core20 branch)
• Run a game (tested with Neverputt, Neverball and Starfighter)
• Close the Mir-on-X window

Expected:

• Window closes and Mircade stops

Actual:

• Window closes but the game's process runs indefinitely and consumes a considerable amount of CPU

[AlanGriffiths]

I'd guess this is because the games ignore a close request and Mir doesn't follow that with a SIGKILL. That is reasonable most of the time, but maybe it should kill things in a shutdown scenario.