Part of the brush assets project followups, see #116337.
Based on feedback, it seems important to indicate to the user when a brush has
unsaved changes.
There's no reliable updating mechanism we can use or hook into here, except for
RNA "update" callbacks. Brush data gets changed in many places in code, the only
way to do this seems manual tagging every time a brush property gets changed.
This PR introduces `BKE_brush_tag_unsaved_changes()` for this. I spent some time
going through all brush properties to ensure changes call the tagging function.
A known limitation with this will be that changes to dependencies won't be
indicated in the brush. E.g. Changing the texture attached to a brush won't make
the brush be indicated as changed.
The UI to indicate the changed brushes is being discussed still, see #128846.
Pull Request: https://projects.blender.org/blender/blender/pulls/128845
d177388979