Implement IMA ADPCM WAV file support

Fixes #4628.

Unfortunately it seems @jamslater lost interest or is otherwise not able to continue working on it at the moment, so I'm trying to bring this into a "good enough" state so we finally have an alternative to AUD with both compression and stereo support.

Code is pretty much @jamslater's work, I just rebased and tried to adress at least a few of @ScottNZ's style nits.
I'll need more specific examples for further improvements in that regard.
There's no potential confusion about the order of operations here, so you can leave the inner parentheses out.