Deleting view group should also delete views

[ianrrees]

Moved over from ianrrees/FreeCAD_tinkering:

Currently, the individual views end up orphaned when a view group is deleted.

[WandererFan]

Using "signalDeletedObject", this is fairly easy if we're willing to get "can't reparent deleted item" warnings from Tree.cpp.

Which is worse: superfluous warnings or a dirty tree?

[ianrrees]

Um, neither of those sounds great to me.

I suppose in the short term, it would be best to get the user-facing behaviour right, then worry about fixing the warnings. But, we could also just leave it as-is until we can figure out a better way.