Tiny change to make grep's life easier: Rename the fields of the menu

Tiny change to make grep's life easier: Rename the fields of the menu
context. Not really worth a ChangeLog entry...

IMHO it looks like we could kill the whole MenuContext stuff, it is of no
use currently and some things look strange, like e.g. having a context per
menu. The latter is not OK when a menu is attached to multiple windows.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@662 7f0cb862-5218-0410-a997-914c9d46530a