Added support for more arbitrary number of mouse buttons (though only the

first three can work with menus), in X.  This closes a compatibility gap
with old GLUT, and leaves us the option of supporting scroll-wheels
properly.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@210 7f0cb862-5218-0410-a997-914c9d46530a