Healing brush tool

Summary:
T3589: Create smart fill brush task. Added new tool to seamlessly fill in areas in the image.
Demo is available here: https://youtu.be/jI87VzDtkPY

Test Plan:
Note that this code runs much slower when compiled in debug mode than in release mode. Please, test release for performance issues.

1. Press the "bandaid" tool.
2. Use brush to paint over the area you want patched.
3. Area should be replaced with reasonable image that blends with the image around it.

Reviewers: Krita, dkazakov, rempt

Reviewed By: Krita, dkazakov, rempt

Differential Revision: https://phabricator.kde.org/D5327