Those property features don't seem to be in use anywhere.
They are redundant since the docs cover the same information
and more. They also created unnecessary translation work.
Closes#4904
Remove a boatload of "or %NULL" from nullable parameters
and return values. gi-docgen generates suitable text from
the annotation that we don't need to duplicate.
This adds a few missing nullable annotations too.
The GtkTreeListRowSortKeys implementation doesn't
know how it wants to cache its keys, and just crashes.
Since that is not cool, add a bandaid fix that forces
it to recreate its keys instead. Extra work, but hey,
no crash.
Related: #3228
