Fix compilation, add CI

[maxgerhardt]

• fixes compile error in cc65-audio (#124)
• updates requirements.txt to not have ancient versions and all actually needed dependencies (Pillow + numpy)
• Add CI to build all demos on a Ubuntu machine with latest emulator
• Add badges to README

cc65-audio demo outputs audio, but in my virtual machine the audio was slightly stuttering -- not sure if the program itself needs to be updated more or if I just have bad performance. For now, I just wanted this PR to have minimal impact on the actual application code that does not go beyond "fix compilation".

I've also tested the compiled programs in the latest emulator at random.

Compiled cc65-sprite demo runs.

Affine demo runs.

Demos that are not running (basically blank screen after some optional intro screen): snake, layer, frog, balls, ...

However, other PRs like #115 and #122 may fix some of these.

Supersedes #108.

[indigodarkwolf]

I fear you could be biting off quite a lot if you plan to go and fix all the demos to work again. Especially crufty items like my "matrix crawl" demo. :3 But thanks for working to get a few going again!