Basic support for large fonts - V2

[roryjamesallen]

This is a copy of all the work done by @francoisgeorgy in #234

I've completed all the changes requested since the most recent commits on the old branch, but there may well be other issues I'm not yet aware of. The custom_font_demo.py script does run and display correctly at least.

[roryjamesallen]

Note: custom_font_demo.py should be removed from contrib before this PR is merged

[francoisgeorgy]

Thank you so much @roryjamesallen for having fixed and finished my work. That's very kind of you.
I still regret not having had the time to do it myself, but family issues and too much work have kept me so busy I haven't even had time to use my eurorack for the last months.

[roryjamesallen]

Thank you! The work was almost completely finished, I only started working on it when I realised you'd done all the hard stuff already.
I hope everything is well and you can get back to your modular soon:)

[mjaskula]

Thanks for putting the work into this @roryjamesallen. I had meant to take it over, but never found the time.

You are right that this seems pretty close to ready to merge. I've identified a couple of documentation changes, as well as some code changes that I think will make the future merge into the firmware easier. Let me know if any of my comments are confusing.

[mjaskula]

I just have one more documentation change to request and an optional suggestion to consider. After that it all looks good to me and is ready to merge from my point of view. Good work.

[mjaskula]

Thanks for those doc changes. Looks good to me now.