As part of the brush assets project, the image editor needs an asset
shelf to display brush assets for texture paint mode. This commit adds
the unused asset shelf region, similar to the 3D viewport's region that
is currently just used for the pose library. For now, users will just
see an "Asset Shelf" toggle in the view menu.
Pull Request: https://projects.blender.org/blender/blender/pulls/121034
c60a1006e5
