Adds support for saving some view state persistently and uses this to keep the
height of a tree-view, even as the region containing it is hidden, or the file
re-loaded.
Fixes#129058.
Basically the design is to have state stored in the region, so it can be saved
to files. Views types (tree-view, grid-view, etc) can decide themselves if they
have state to be preserved, and what state that is. If a view wants to preserve
state, it's stored in a list inside the region, identified by the view's idname.
Limitation is that multiple instances of the same view would share these bits of
state, in practice I don't think that's ever an issue.
More state can be added to be preserved as needed. Since different kinds of
views may require different state, I was thinking we could add ID properties to
`uiViewState` even, making it much more dynamic.
Pull Request: https://projects.blender.org/blender/blender/pulls/130292
f0db870822