fix(input): overlapping modifiers not consistently respected (#116)

Bug #115 complains that the player couldn't set the unequip modifier
and the utility-activation modifier to the same thing and have them
consistently respected. When I looked into this I was horrified to
read what a mess I'd made of tracking key input.

It's still a mess after this PR, but it's less of a mess. And this
fixes #115.

We demote the hotkey enum to a supporting role instead of a central
one. The controller uses the keycode as the identifier for tracked
keys, since it is in fact unique while hotkeys, as noted above, might
not be. The modifier hotkey variants have been collapsed into a single
variant with internal data representing what modifiers might be
associated with that keypress (sadly via a new enum, `Modifier`). The
controller passes the tracked key struct down through key event
handlers when needed, instead of the hotkey variant. It also uses the
`Action` enum--which is definitely staying around--when that is
meaningful.

I didn't get to nuke some enum types from orbit yet, but I will soon.