support i2c gpio expanders

[eric-humane]

Allows for things like https://www.sparkfun.com/sparkfun-qwiic-directional-pad.html to be used. I have tested in my own project and it works great.

[Spirik]

Hello there!

This looks like a really great addition! Although I have zero experience working with gpio expanders to be completely honest, so I have to trust you on this=)

Hopefully I will get some time to look closely at the code on the weekend (for some minor codestyle tweaks and documentation additions). And I will definitely have to get me some of the hardware to test it properly sometime in the future:)

One question for now: does simultaneous presses work correctly with this (i.e. in accordance with timing diagrams)? Given the example you provided they should, but just checking to be sure.

Thank you for your contribution! 🤝

[eric-humane]

I will double check!

[eric-humane]

Sorry, I still haven't double checked. It appears that there are many variations of GPIO expanders with many different capabilities, that said I have been using this PR with one of these:

https://learn.adafruit.com/gamepad-qt

Which is based on Adafruit seesaw and seemingly allows multiple readings at once, where some other GPIO boards (like in my examples) aren't capable of reading all the registers at once (or perhaps just lack the necessary API interface to do so).

[Spirik]

No worries! I still haven't get myself an expander module for testing either. Just got my hands on SparkFun Red board with qwiic connector, but not the actual button/navigation module.

I wonder if it is possible to implement reasonably universal support for these, without relying on specific third-party library (like the one provided by SparkFun).