Making more keys work and the code a little easier to follow.

Main changes:

* Adding defines for the NeXT modifier keys.
* Making it so all keys work, some through horrible hacks.
* Switching the Control and Alt mappings so it matches what is on the keyboard.
* Adding mappings for the volume/brightness up/down to standard keys (ins, del, pgup, pgdown) since the MouseKeyboard Arduino library doesn't have a way to send those as anything more useful.