Implemented fullscreen and display settings for OS-X

This implements the following functionality for OS-X:
  window_set_fullscreen()
  window_get_fullscreen()
  display_get_width()
  display_get_height()

Switching to fullscreen and back to windowed will occasionally cause errors (especially if Views are involved), but most games only set fullscreen once. Moreover, the previous function skeletons didn't work at all, so I feel this is a step forward.