[sugar] Killing hung activities

[Bernie Innocenti]

Today I was testing Paing in Joyride and managed to hung it in a
way that hogs the CPU.

There seems to be no way to kill such an activity from Sugar.
Stop just tries to close the X window, which is ineffective in
such cases.

We'd need to fire a timer to check if the activity is still there
after a few seconds and, in that case, send a SIGKILL.  A safer
design would pop a Wait/Force Quit window before proceeding.

-- 
   \___/  Bernie Innocenti - http://www.codewiz.org/
  _| X |  Sugar Labs Team  - http://www.sugarlabs.org/
  \|_O_|  "It's an education project, not a laptop project!"