Currently, depending on whether or not a brush asset is part of a
library, the context menu shown for it changes significantly.
To better support local brush asset modifications, and make the UI more
consistent, this commit exposes the following operators:
* `brush.asset_edit_metadata`
* `brush.asset_load_preview`
* `brush.asset_save`
* `brush.asset_revert`
Of these four operators, the first two are allowed to execute, whereas
the latter two are always disabled on local assets via the corresponding
`poll` methods, as neither is applicable for this usecase.
Related to #138105
Pull Request: https://projects.blender.org/blender/blender/pulls/138376
2e3debc4ca