This implements the "boundary strokes" features (from 4.2 LTS).
To make it a bit more clear, the feature is now called "fill guides".
It uses the same `Alt`+`LMB` shortcut.
Fill guides are denoted by a `".is_fill_guide"` attribute set to `true`.
Otherwise they behave as normal strokes.
There is the `Clean Up` > `Remove Fill Guides` operator to remove
the strokes.
This also adds an option to the Fill tool to automatically remove fill
guides once a fill has been created.
Pull Request: https://projects.blender.org/blender/blender/pulls/139293
d332411b21