Add mouse button constants

[MarkOates]

Mouse button numbering can be confusing. Adding constants could help.

This PR adds ALLEGRO_MOUSE_BUTTON_LEFT, ALLEGRO_MOUSE_BUTTON_RIGHT, and ALLEGRO_MOUSE_BUTTON_MIDDLE. This would make code like the following available:

switch (event.mouse.button)
{
   case ALLEGRO_MOUSE_BUTTON_LEFT:
      printf("left button pressed!\n");
   break;

   case ALLEGRO_MOUSE_BUTTON_RIGHT:
      printf("right button pressed!\n");
   break;

   case ALLEGRO_MOUSE_BUTTON_MIDDLE:
      printf("middle button pressed!\n");
   break;
}

Additions/revisions to the mouse and events documentation are included as well as disclaimers about the possibility of more/fewer mouse buttons. Also, a typo is fixed in docs/src/refman/keyboard.txt.

Some questions

• Do I need to "make" the docs?
• ALLEGRO_MOUSE_BUTTON_UNDEF is not included, should it be?
• There may be other example programs that could be updated to include the constants. Should we worry about including that?