Add a rectangle of influence for 2pp assistant and vanishing point assistant

I added a limited area (a rectangle) to the 2pp assistant (of course the user can choose if they want it or not). I still want to add it to the vanishing point assistant, which will allow the user to use the assistants for comics exclusively: it would add both the 3pp and 1pp availability (because 2pp can be used for 3pp if you disable vertical line).

I'd like input on user stuff - the behaviour, Tool Options etc.

In the future, I'd want to have an option to convert 2pp to 2pp local and back with a simple button click on the Tool Options. It's a bit more complicated GUI-wise so I left it as a creation-only option for now.

Test Plan

Example test file: 2pplocal_6.kra

• Open test file and play around.
• Create a new file, and create one or more 2pp local assistants. (Note: I'm pretty sure the thicker lines for the second and next assistants are a bug already present on master).
• Play around.
• Save and then reopen the file: assistant should behave the same way as before.
• Save to an assistant set, then create a new file and load the assistant set. It should work just like before.

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.