The complexity with model items vs row items is really confusing. Add to
that treelistmodel position vs child model position vs parent position,
and you're so confused, even the best naming can't help.
And once you're there, consider passthrough vs non-passthrough...
This patch does multiple things:
1. Add a custom persistent per-row object.
2. Move all per-row API to that object. This means notifications are now
possible.
3. Add a "passthrough" construct-only property to the TreeListModel that
influences if the model returns these new object or passes through
the ones from the model.
This greatly simplifies the code needed to be written for widgetry,
because one can just connect the per-row object to the expanders that
expand and collapse rows.
As an added power feature, these objects can also be passed through
further models (like filter models).
It also adds kind of a hack to Adwaita to make the test look neat.
