Removing a state does not explicitly remove its state elements

[franzlst]

When removing a state in the core (by calling remove_state on its parent), all connections to the state are explicitly removed and also the state itself.
However, the state elements (outcomes, data ports) are not explicitly removed. I think this could be problematic, as references and models to these elements could be maintained. Currently, the situation seems to be fine, nevertheless the behavior should be changed. This should be done with care, to avoid any side effects.

Originally created by @franzlst (Franz.Steinmetz@dlr.de) at 2016-01-11 14:09:37+00:00 (moved from RMC internal repository)

[franzlst]

Improve widgets to work with destruction method. So issue is closed with commit e4bf92cb2a3d4f6ae.

Originally created by @Rbelder at 2016-05-23 13:28:30+00:00 (moved from RMC internal repository)