This also replaces Github workflows with .fmsbw CI scripts.

* Docker turned out to be impractical since you cannot connect a dockerized Jack client
  to a Jack server on the host as library versions would have to match exactly.
  Neither did I get Netjack to run in Docker.
  Also when running in Docker, it would be tricky to get gnuplot to work (and Tcl/Tk support in the future).
  Especially the Jupyter notebook packaging would have been a lot easier, though with Docker.
* AppImage will automatically avoid bundling libjack and require a compatible libjack on the host instead.
* Using pkg2appimage ensures we can pull in dependencies as Debian packages.
* This version of AppImage.yml does not contain Jupyter notebooks.

* this is unreliable but should be tested on the build server as well
* drops the lua-discount dependency
* updated TODO

* This allows the native syntax `for f in Stream:iter() do ... end` without using lambda
  functions.
  Also you can use `break` and `return` statements.
* On the other hand we cannot exploit the extended xpcall() semantics and had to introduce
  another lambda in Stream:play().
* In general the number of function calls per tick stays the same.
  Stream:gtick() itself could be used as an iterator, but Stream:iter() adds checking for CTRL+C,
  resetting of the sample cache and binding functions.