[AD] support for toggling fullscreen

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

As I said in the ALLEGRO_FULLSCREEN_WINDOW thread already, all Linux
apps I have here (at least the ones using either GTK or Qt) allow
toggling between fullscreen and windowed at any time and they do so
instantly and without changing monitor resolution. It's something A5
apps can't do right now.

I also tried in OSX - there the standard applications don't seem to
allow toggling to fullscreen - however if I play a youtube video and
click the toggle fullscreen button it does the same instant-switch as in
Linux, without ever changing monitor resolution.

No idea about Windows since I don't have access to one but I assume it's
possible there as well.

So I think we should add the following things:

* Switch an existing window between fullscreen and windowed by changing
monitor resolution (like ex_fs_resize it would probably be awkward with
the screen flickering and other monitors blanking out and so on...)

* Switch an existing window between fullscreen and windowed without
changing monitor resolution (resizing the window instead). This likely
will be instant also under Windows and not have any side effects.

Could either use two functions or if we use the display flag toggling
function just toggle the ALLEGRO_FULLSCREEN flag for the fist kind of
fullscreen toggle, and ALLEGRO_FULLSCREEN_WINDOW (or
ALLEGRO_WINDOWED_FULLSCREEN? or any better name?) for the second.

The implementation for Linux is already done and I can probably do it
for OSX if nobody else wants to. Windows could just ignore it for now
and not allow toggling until someone figures out how to code it.

-- 
Elias Pschernig <elias.pschernig@xxxxxxxxxx>