demos: Add support for a Slint demo

[alistair23]

Demo application using Slint and midipi to display a GUI on a st7789

TODO

Depends on #549 otherwise the build fails as we run out of flash

[alevy]

This is starting to get pretty cool!

I think a very good next target would be to include port something like this to using the Screen driver, rather than the midipi-over-SPI stack as well.

Worth maybe noting somewhere that this is all fine with the Slint community license (which distributes Slint under GPLv3 and is compatible with Apache/MIT, which covers including it in this repo, but not necessarily other projects which choose not to distribute under a compatible source license.)

[alevy]

(Not sure how to rerun the CI checks now that #549 is merged)