[AD] intercepting Windows messages

[Matthew Leverton <meffer@xxxxxxxxxx>]

Attached is a patch that adds two functions under Windows that allows
the user to intercept messages sent to the window.

This would allow for things like adding a system menu to the display window.

### API: al_add_win_window_callback

The specified callback function will intercept the window's message
before Allegro processes it. If the callback function consumes the event,
then it should return true. In that case, Allegro will not do anything
with the event.

Returns true if the callback was added.

### API: al_remove_win_window_callback

Removes the callback that was previously registered with
[al_add_win_window_callback].

Returns true if the callback was removed.


--
Matthew Leverton

Attachment: wincallback.diff
Description: Binary data