Various Comic Panel Editing Tool fixes

• Move performance checks in Knife Tool to functions (just cleaning code, though it also should prevent unlucky potential bug with the newlySelectedShapes)
• Implement nicer behaviour in cutting comic panels
• Better memory management in Cut Through Shapes in Knife Tool
• Fix superfluous undo entries for Knife Tool
• Fix Knife Tool not removing covered shapes
• Fix Knife Tool inaction for perfect mouse lines

The "nicer behaviour" is here, option A:

Test Plan

1. Try to cut with Shift and match the cursor with the little dot at the end of the line. It should work normally.
2. Make the gap very big, so big that it covers whole shapes, and try to cut through them. They should disappear completely.
3. Try to cut outside of the shapes, in a way that no shapes are cut. Check that there are no undo entries created.
4. Check the behaviour shown above on the image.

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.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.