Upgraded RtMidi to version 2.1.0

Hello,
I upgraded the internal RtMidi library from version 2.0.1 to version 2.1.0. I used the official library published into the official RtMidi website from Gary. I tested this upgrade in both Linux and Windows without issues. I don't have a Mac so I couldn't test in that environment, however I don't expect any problems.

Cheers,
Hugo.