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)

[inesmaria08]

Hi! I started working on a screen driver API implementation. @alistair23 @alevy just wanna make sure we don't duplicate work, please let me know if you have something in progress.

[alistair23]

Nothing from me

[alevy]

@inesmaria08 nothing from me at the moment, though it would be have been some near on my todo list, so awesome that you're doing it! I expect this to be pretty straightforward, but n.b. my recent changes in tock/tock#4206 and tock/libtock-c#469 w.r.t to swapping endianness for 16-bit color.