Add touch/mouse support

[icculus]

So this will need some basic graphics for clicking/tapping (directional arrows and a sword button...we can just count a tap on the screen during the intro as the start button), and decide whether C or Lua is in charge of managing these.

Mouse cursor should be disabled when another input type (controller button pressed, key press, or a touch) happens. Controller axes aren't enough, it has to be a button; likewise we don't reenable the mouse cursor unless there is an actual mouse button press.

Notable that even if the user isn't using the mouse, having graphics that can light up as if pressed is useful for adding a tutorial mode, where we light up moves when they are valid. That's a future goal.

We don't need relative mode or mouse grabbing.

Touch can probably be mapped to a left mouse button and reuse all the code.

[icculus]

Turns out RetroArch on mobile will provide on-screen controls to simulate a gamepad, so this is less urgent.