Operations on multiple layers/groups

This MR is under GSoC'21 Project: Operations on multiple layers/groups. For more info, visit: https://phabricator.kde.org/T14546

The final commit before GSoC pencils down: !888 (10ed0e77)

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.