With the release of the brush assets project in 4.3, most brush
management moved out of the current .blend file into either the
essentials library or user-created libraries.
With this change, a number of workflows became more difficult:
* Handling a large library of texture and texture properties for brushes
due to ID linkage constraints.
* Having local tweaks to a brush without bloating the the asset library
and reducing discoverability.
This commit introduces an intermediate step to assist with both of the
preceding pain points. The `brush.asset_save_as` operator is extended
to allow saving into the current blend file via the `Duplicate Asset`
context menu entry.
Additionally, these features help ease authoring brush assets in general
from the UI instead of requiring manual datablock management.
This allows brushes to be stored alongside other data inside a specific
blend file.
Related to #129655 and [1].
[1] https://blender.community/c/rightclickselect/XYMA/
Pull Request: https://projects.blender.org/blender/blender/pulls/138105
4ecde8dc53