Magnetic Lasso for Krita

Added Magnetic Lasso to Krita. Though it works, it has multiple places where it could be improved,

• Cursors, we need some better cursors, 😛
• Ctrl-Z undoes the stroke if there is already a selection, according to Dmitry I need to refactor the selection tools to fix this
• Use a better filter, like the Canny filter
• Compress mouse signals
• Use caching instead of filtering the whole Paint Device at once

Test Plan

Open Krita, use the Magnetic Lasso to your heart's content.

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.