Add support for font guidelines

[ollimeier]

Fixes #1963 #909 #2026

This allows adding/editing of font level guidelines via the sources panel.
Currently it's not possible to edit font level guidelines within the editor view (we may want to create a new issue for this and see it as a follow up).

Quick demo:

Screen.Recording.2025-02-12.at.11.17.42.mp4

[ollimeier]

So labels containing descender letters are poorly aligned

Instead of getTextHeight(), we should consider a function getVerticalCenter(), that returns (metrics.actualBoundingBoxAscent - metrics.actualBoundingBoxDescent) / 2 (given that actualBoundingBoxDescent is a positive value for a distance below the baseline)

OK, I have reworked it in this PR. Done.

[ollimeier]

@justvanrossum I am done with this and I hope we can be merged soon.
I have not changed the icon to a lock, because it does to feel right. It is technically wrong and also just a temporary thing as we want to be able to edit the font guidelines in the editor view. But if you have a different opinion on that, I am happy to change it. Just let me know. It's easy implementing it.