[AD] new proposal for *_is_* functions

[Peter Wang <novalazy@xxxxxxxxxx>]

I had another think about how to name the "is" functions.
Most of them are rarely going to be called (if ever), so consistency
should be the deciding principle.  al_event_queue_is_empty is the only
one of the functions which will be commonly called.

Many of the functions could/should be handled differently, sidestepping
the issue:

* al_is_opengl_extension_supported -> al_have_opengl_extension
    It's shorter and matches al_have_d3d_*

* al_is_sub_bitmap -> remove
    A bitmap flag would be consistent with ALLEGRO_MEMORY_BITMAP, etc.

* al_timer_is_started -> al_get_timer_started
    Matches the other al_get_timer_* functions

* al_fs_entry_is_directory -> remove
* al_fs_entry_is_file -> remove
    Conveniences for al_get_fs_entry_mode only.

* al_is_path_present -> remove
    This contains a workaround for the trailing slash problem on
    Windows, but that should be done in fshook_stdio.c.
    Then al_is_path_present would be nothing more than
    al_filename_exists(al_path_cstr(path, ALLEGRO_NATIVE_PATH_SEP));

* al_event_queue_is_empty -> al_is_event_queue_empty

* al_is_joystick_installed
    Currently missing.

Peter