Add new ID_IS_EDITABLE macro that checks if the ID can be edited in the
user interface. Replace usage of ID_IS_LINKED where it is used with this
meaning.
Also add a corresponding ID.is_editable property for Python.
This prepares for the ability to edit some linked datablocks for brush
assets.
Pull Request: https://projects.blender.org/blender/blender/pulls/121838