Construct pure virtual methods in inherited classes

[rcalixte]

This unfortunately ends up as a sort of breaking change. Now that the base implementations are defined, they are returning 0 in some cases. The result is that not setting an override means a 0 value being returned for something like the column count, leading to an empty-looking model where Qt's private default may return 1 (in the case of QAbstractListModel but not QAbstractTableModel). The fix is mechanical but it's still a breaking change. I've updated the examples that I could find that were impacted.

While it's not strictly marshalling in the purest sense, the tests are for validation that can otherwise only be seen visually. Would it make sense to split the tests into a new testing nomenclature?

• Closes Panic "can only override virtual methods for directly constructed types" when calling a parent class override method #246

cc @arnetheduck

[rcalixte]

I think I'm going to close this in favor of #341. I believe these methods are not meant for overrides and that we just need to address the panics in order to resolve #246.