Draft: Add concentric ellipses to Perspective Ellipse Assistant

• Saving and loading
• Generic ellipse + hyperbola + parabola formula (so it works closer to the horizon)
• Mirrored image on the other side of the horizon
• Brush works on the hyperbolas and mirrored side too
• Uses ellipse when possible (for speed?)

Test Plan

1. Create new document.
2. Add a Perspective Ellipse assistant.
3. Check "Concentric Ellipses" in Tool Options.
4. Switch to brush, use the concentric ellipses.
5. See that the ellipses become hyperbolas when you move towards the horizon, and on the other side of the horizon, it looks like the same assistant mirrored.
6. Save the document, open and see that the perspective ellipse is still there and it's concentric.

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.