Fixed not working step-into/-over/-out

Some async notifications were not parsed correctly.
Added test to verify that in the future.
Fixed crash on second debug session

Caused by not unmounthing some GDB events and pointing to old instance of
DebugEngine.
Added more comments about GDB configuration
Renamed AD7Engine state and its enum type
Removed old GDB testing code
Refactored some method names of EventDispatcher
Ending DE session in case of unexpected GDB exit

For example: ntoarm-gdb.exe crashed or process killed by developer.
Otherwise the debugging session would hang and start showing strange
error messages.