Add assignable media and other buttons

[ho0ber]

From discord:
I do think I'll make it possible to rebind the media buttons however you like at some point. We'll set up reasonable defaults, but then if someone really wants a media function (like putting next on the record button) it's easy to change.

That'll also allow folks to make better use of all of the assorted buttons that really only apply to use in a DAW and not for most our uses - I'll probably do the basics like "launch a file", "lock computer", "sleep", "quit" but then also add options for explicit ways to save and load config because I know that once I have a config set up I probably don't want to wipe it out because I bumped a config button. I'll just have an "auto-save" feature on by default, but make it easy to turn off and bind save/load options