Debugger: Make sure stateChanged is called from debugger thread

Make sure stateAboutToBeChanged(), stateChanged() is always called
from the debugger thread. This matches how messageReceived()
is called. On exit, run an event loop until all stateAboutToBeChanged
calls have returned.

Change-Id: I9cd6199cc80552ad97e4b7d504ea91aa116a6a34
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>