treelistmodel: Refactor to add GtkTreeListRow

treelistmodel: Refactor to add GtkTreeListRow

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.