as per efd3758f6a strcasecmp() is not a C
standard thing (not that we bothered including any header for it anyway)
and so this test failed to build on Windows with Microsoft Visual C.
This is way more complicated than it should be, because it requires
manually limiting the number of open file enumerators.
On the other hand, it exhaustively tests the items-changed emission of
all involved listmodels because those signals come in pretty much
randomly.
It's also 50% slower than the sync version, with the caeat that the sync
version only shows the UI after it's done loading, while this version
shows it right away.
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.
