Better default instruments

[SuperDisk]

Proposed by @RichardULZ in #5.

[...]Some better named starting preset instruments would go a long way.

Such as the 3 square duty types 12.5, 25, 50, and same 3 duty types with an envelope. Wave samples such as the 3 squares 12.5, 25, 50% (allows quick reusable tracks!) triangle, sine, sawtooth, others.

[SuperDisk]

I added a few default instruments for the Square and Wave channels in commit f9fe74e. I'm going to wait until I redo the noise channel to have linear scale until I add default instruments for those.