Opening split view when structural view is not the current view doesn't work properly

[Matthias Schoettle]

Steps to reproduce:

1. Load the Observer aspect.
2. Switch to the view of message views.
3. Enter the split view for ZeroToMany.

Result:

On the upper part of the screen, the view for message views wasn't hidden. The structural view of Observer is placed on top. After closing the split view both views stay.

This happens, because the split view always assumes that it is entered from the structural view. It might not be the case. Therefore, either don't assume, but handle doing it from any view or only allow the user to enter it from the structural view.

[Matthias Schoettle]

References issue #71. Preparation for issue #71.

DisplayAspectScene keeps state of the current and previous displayed views. New methods allow to switch to a certain view, switch to the previous view and get the current view.

Operations and state specific to the instantiation split view is therefore now obsolete and was removed.

→ <<cset c27f2d9c98eb>>

[Matthias Schoettle]

Resolved issue #71: The enhancements from commit c27f2d9 already fix the main problem. The previous view is displayed when the split view is closed.

However, certain cases needed to be addressed: When the structural view wasn't the previous view and the split view gets destroyed, this should not destroy the structural view (one of its children at that point). Furthermore, when the split view is enabled, switch to the structural view in the lower-level aspect scene in case it wasn't displayed before. Otherwise the structural view will be shown after (because it is the previous view at the beginning).

→ <<cset 6f147d359f8e>>