Make X-Input-Method into an enum

[Miss-Inputs]

Ideally we'd be restricting it to a specific set of values. Everything else can be "Custom".

This relies on doing #24 first though, if not simply because that'd be the best way to figure out what all the possible input methods are.

I'd also need to figure out the difference between a dial and a paddle. Still haven't really done that. It is a mystery.

(Once I've done that, maybe I can autodetect arcade games that actually have steering wheels and not really paddles.)

[Miss-Inputs]

Dang! Ended up finding out everything I need to know. It's all there in https://github.com/mamedev/mame/blob/master/src/emu/ioport.cpp

And also https://github.com/mamedev/mame/blob/master/src/frontend/mame/info.cpp, which I need to refer to more often.

• "stick"
  • Absolute analog device, autocentering
• "paddle"
  • Absolute analog device, autocentering
• "pedal"
  • Absolute analog device, autocentering, starts at min range and autocenters to min range
• "lightgun"
  • "Absolute analog device, doesn't autocenter
• "positional"
  • Absolute analog device, can wrap around like a relative device
• "dial"
  • Relative analog device
• "trackball"
  • Relative analog device
• "mouse"
  • Relative analog device
• "joy"
  • 1 digital directional input (maybe with buttons)
• "only_buttons"
  • Buttons but no directional input
• "doubejoy"
  • 2 digital directional inputs (maybe with buttons)
• "triplejoy"
  • 3 digital directional inputs (maybe with buttons)
• "keypad"
  • Effectively same as keyboard, except generally only with a few buttons
• "keyboard"
  • Alphabetical keyboard
• "mahjong"
  • Specialized keyboard for mahjong
• "hanafuda"
  • Specialized keyboard for hanafuda
• "gambling"
  • 5 buttons in a row, which happens to be used for gambling machines. Well, it can have more... okay, it's a bit confusing. Let's just call it Custom for our purposes.