Added preliminary support for the SDL2 touch api. Missing items:
  1. Timestamps
  2. TouchID (the Device ID)
      As inline with the SDL spec, we will pass a touch ID of
      `SDL_TOUCH_MOUSEID` for touch events that are simulated by the
      mouse so games can rely solely on touch events if they like.

Includes the SDL2 Copyright notice on the headers that contain SDL2
content.
Includes a fix to SDL_PeepEvents.