Debugger: Refactor run control termination.

Fix breakage introduced by the new asynchronous stop() methods
of the debugger run controls. Allow for RunControl::stop() to
be asynchronous by introducing a return enumeration indicating
that. Introduce additional method aboutToStop() asking user
to quit (tie that to the RunControl instead of having to hack
the behaviour elsewhere).
If asynchronous stop is detected, terminate the ProjectExplorer
asynchronously.
This makes the behaviour consistent across switching sessions/
closing outputwindow tabs and quitting Qt Creator.

Reviewed-by: dt
Rubber-stamped-by: hjk