Snapshot Docker

This is a snapshot docker that allows users to create snapshots (shallow copies) of the current document, and switch back to any snapshot at a later time. Users can also edit the names of the snapshots or delete them in the docker.

Test Plan

0. Create a document.
1. Paint something.
2. Add some guides.
3. Add some assistants.
4. Add some reference images.
5. Add a document-wide palette by creating a palette in the palette docker and then click "edit this palette" then click "OK."
6. Crop the image to a smaller size.
7. Create a snapshot in the snapshot docker.
8. Double click that snapshot to rename it.
9. Restore the document to the initial state in the History Docker by clicking <empty>.

Everything in the document should return to the state before creating the snapshot.

TODO

0. Assistants decoration needs to be updated correctly.
1. We probably should use counts for snapshot names.
2. [Disputable] Should undo stack be cleared, or should we add an undo command for switching snapshots? NO

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!) It breaks one more image test than master: kis_node_model_test.
• 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.