TODO: Fix events

[Lucretia]

This is two issues in one:

• Need a tagged event hierarchy rather than a copy of the C struct.
  • Root type
  • Keyboard type
  • Joystick type
  • Mouse type
• Rename SDL.Events.Events and make it a private package, or move the Events type to the private part of SDL.Events.
• Move SDL.Events.Events.Poll to SDL.Events.Poll and change to use the tagged types.
  • Use Ada's factory for creating tagged types - too much overhead, too slow?
• Investigate calling SDL.Events.Poll in the library, copying all events to an event queue held in a protected object. This would help with tasking at a higher level. This might also be a bit slower, needs to be checked.