Controller support and Global keys

[niamu]

Resolves #1978 regarding global keys for selecting and going back.

It's not a perfect solution as ENTER will now always function as JUMP. But that's not exactly a problem.

This change also starts to cleanup some dead code and add the beginnings of the structure necessary for gamepad control support (#2356).

[niamu]

No analog stick support at the moment, but otherwise, very usable.

[8bitgentleman]

First off this is dope, the game feels much more natural when played with a controller.
Alright I've tested my Dualshock 3 and USB SNES controllers with this. The PS3 controller works perfectly as long as it was correctly set up in the OS. I forgot to set it up at first and it kept disconnecting and reconnecting which turned out to be my fault not Hawkthorne's. The SNES controller does not work. This is because the USB SNES controllers identify themselves as joysticks not gamepads. Love sees gamepads and joysticks as different things apparently.

[niamu]

I have no idea if this latest change is sufficient to allow for SNES support. My guess is that it won't register button presses on a controller that identifies as a joystick rather than a gamepad, but I can't test that.

[8bitgentleman]

Your guess is correct, the SNES keypresses do not register.

[niamu]

@8bitgentleman, when you have the time to join me on IRC, we can walk through some debugging together and figure out how to finish this up.

[niamu]

We have controller support now for Xbox and PlayStation controllers as well as the mapping for the Retro SNES controller. No analog stick support as of yet, but the game is playable, so we're merging this in now.