All the list model tests were leaking items,
because g_list_model_get_item is transfer full.
Fixing these unveils a crash in the treelistmodel
and maplistmodel tests.
The calls used old bugzilla URLs and nobody cared about that.
So apparently they are very unused.
There's also a potential conflict between gitlab and bugzilla URLs and
what base bug to use there.
The old usages have been converted to comments.
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.
