Allow typewriter theming, add typewriter clear button, add piano outline colour

[prayerie]

Allows the typewriter to be themed. I edited the texture for the typewriter to add a couple of new colours (which are still black) to the palette so the outline and key labels can be set independently.

Also adds a "clear" button which just sets the typewriter's text to an empty string. (closes #190)

Also adds another colour for the piano's outline. I have updated some default themes to also use this new typewriter theming, here are screenshots:

Sky

Velocity

Sand

Neon (which I also gave custom piano key colours)

Concrete

bye2theme

Bubblegum

[asiekierka]

Thank you!