[AD] display options again

[Peter Wang <novalazy@xxxxxxxxxx>]

I think we can do better with the display flags/options stuff,
as has been raised before.

Display flags could be integrated into the more general display options
framework.  The options API is a bit harder to work with, though,
so maybe we have to do something about that.

- ALLEGRO_WINDOW/FULLSCREEN/FULLSCREEN_WINDOW could be replaced by a
  single display option, with three allowable values

- ALLEGRO_OPENGL/ALLEGRO_OPENGL_3_0/ALLEGRO_OPENGL_FORWARD_COMPATIBLE
  may be similarly collated (not sure)

- ALLEGRO_GENERATE_EXPOSE_EVENTS should be a display option

- ALLEGRO_MINIMIZED could be replaced by a display option, perhaps one
  that accounts for other window states as well, e.g. hidden? iconified?
  swallowed?

- new_refresh_rate should just be a display option

- al_set_current_adapter could be replaced by a display option
  (it's not TLS right now)

- new_window_position could possibly be replaced by a pair of display
  options, but that's pretty ugly
  (it's also now TLS right now)

- al_store_state doesn't know how to save new display options, only
  flags.  This seems like simply an oversight.

Peter