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