Paste into Active Layer (T14166)

Details on this feature can be found at:

https://phabricator.kde.org/T14166

The page above contains a basic example, and describes the target user group, which workflows are simplified, some interaction details, and a description of what is out of scope.

Since this is an optional feature, this initial version does not try to optimize performance or support all transform preferences: the paste result is always fully composited with the target layer in real time, without a temporary overlay. After completion, the Transform tool stays active.

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.