Copy-On-Write Vector Layers

This MR makes the KoShape hierarchy implicitly-sharable.

Test Plan

I'm not really sure on the steps besides the unit tests, but Krita's functions on vector layers should work the same as before. In particular, cloning and editing the shapes and/or layers should give out the same results as in master.

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.