The data-block selector filters out local data-blocks when trying to
assign to linked data. A lot of data-block management relies on this.
Since these data-blocks are created for the use with brushes only,
create them under the same library that the brush is from, as indirect
data. The UI allows selecting these data-blocks then. We do the same for
textures attached to a brush, and some other data-block types.
Also add paint-curves to the editable data-block types, so they can be
edited despite being considered linked data.
Pull Request: https://projects.blender.org/blender/blender/pulls/130184
a0b2704202