added glutPositionFunc callback, now just need to implement so it does

added glutPositionFunc callback, now just need to implement so it does
something
For all callbacks now using function type definitions from
fg_internal.h.
Reorganized and cleaned up fg_callbacks.c so that there are no wrong
comments in there (timers are global, not per window) and so that all
global, per menu, and per window callbacks are grouped together

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1474 7f0cb862-5218-0410-a997-914c9d46530a