Changes:
- Add do-nothing handler for NES DPCM feature type.
- Note type is now always set at the beginning of each pattern (except for ch.3), seems to prevent "leftover envelope" bugs.
- Support duty cycle patterns using the Duty/Noise macro. This macro must have a length of 4.
- Unique error message on passing a file without a title.
- [GUI] Change icons of "Save" and "Convert".
- [GUI] New help screen.
Built using Nim 2.0.2 (--mm:arc).