Add Lineart Detection

[Anonymous]

This feature will allow the user to complete colour entire lineart zones with the bucket fill.
Like this:
https://girinstud.io/log/wp-content/uploads/2019/02/Peek-2019-02-16-15-28.webm

The feature is already implemented on GIMP.
And I don't see why Sketchbook won't have it.

IT COULD SAVE TONS OF TIME!