Part of blender/blender!125449.
Since brush assets were merged, the brush asset selector and some other brush
UIs wouldn't show up for the primitive tools. This is addressed now. The
primitive tools use the same brush as the brush tool, although only the draw
brushes will work properly. !125449 addresses this so the primitive tools
remember their own draw brush, separate from the brush tool.
Turns out that these primitive tools were never tagged as using brushes by
setting the `data_block` member (or setting the `'USE_BRUSHES'` option since
b64bf66257). The UI was just hardcoded to display things like the brush selector
for primitive tools. So as far as the tool system knew, these tools did not use
brushes.
Tagging the tools properly exposed some issues that are addressed here (see PR
for details).
Pull Request: https://projects.blender.org/blender/blender/pulls/127204
65fdb3fa30