move libADLMIDI out of the codebase

Unless there is a good reason, libADLMIDI should be pulled in as an external rather than being version controlled as part of the Shockolate codebase.

It looks like libADLMIDI source is available right here on GitHub: https://github.com/Wohlstand/libADLMIDI

Edit: Or is it this one? https://bisqwit.iki.fi/source/adlmidi.html - which appears to have a GitHub mirror here: https://github.com/bisqwit/adlmidi