Since 660867fa00, having tree-view items uncollapsed by default using
the `set_collapsed()` functions wouldn't work anymore. An attempt to do
this would assert even, so eb71d9f7bc disabled the assert.
I think a function designed to handle exactly this is the best solution,
it makes the intent & behavior more clear than before, and highlights
that this is a special case.
Mitigates #117957, in that it solves the regression, but tree-views still
don't remember their state on screen-layout changes. This is a known
limitation and not supported.
Pull Request: https://projects.blender.org/blender/blender/pulls/119166
745fd2a2cb