Option to kill RAFCON if execution cannot be stopped

After stopping the execution, it might happen that a thread does not join. If RAFCON is closed, the prevents the process from being stopped.

If the execution is still active after a certain amount of time, the user should have the option that the process is killed (`os.exit()`).

added by @Rbelder:
* [ ] so a dialog should be open if the engine is not stopping within a certain duration which offers
* to let the thread and process life (but with a note how to kill it in the console later)
* or to directly kill it by `os.exit()`
* [ ] additional sub-task would be to check if the same interface is possible for core only and to implement it in e.g. the interface-module which is in core and gui sub-package

Originally created by @franzlst (Franz.Steinmetz@dlr.de) at 2018-05-02 11:29:15+00:00 (moved from RMC internal repository)