Rather than storing asset shelf types only for the space type defined
with `bl_space_type`, store them statically/globally so that asset shelf
popups can later be displayed in any space. In earlier designs the asset
shelf was more space dependent than it is now.
For example the brush selector asset shelf popup being implemented for
the brush assets project should be available in the tool settings UI,
which is also displayed in the Properties. If the asset shelf type is
space dependent, this popup won't work in the Properties.
To be clear, `bl_space_type` will still be what defines where the asset
shelf region will be available. But the popup version can be displayed
anywhere.
Pull Request: https://projects.blender.org/blender/blender/pulls/121411
ff4fc03023