Add "boundary fill/select" option to the fill and contiguous selection tools

This MR adds a new option to the fill and contiguous selection tools that uses the "boundary fill" algorithm (instead of "seed fill") to fill/select everything until a specific boundary color.

It just uses the floodfill with some "select until color" selection policy I made for the enclose and fill tool, so the code changes are minimal, mostly ui.

NOTE: If this is aproved, I think that the function names in KisScanlineFill should be revised, they start to look a bit odd, specially the color boundary vs boundary selection.

boundary_fill_01

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.