Added Soundfont Rendering

Hello!

This update adds soundfont (SF2) rendering of MIDI files. You can render a WAV file at the same time your render the MIDI file by checking the "Render Wav" checkbox in the MIDI settings and a valid SF2 soundfont.

There are also bugfixes and a lot of internal rewriting of the source code (but hopefully you won't notice any of that!)

Thanks for all the support!