timer demo now has a menu to:

- set flicker tempo
- test whether menus can be modified in the callback (there was a regression with this in freeglut 2.8.1 that was perchance already fixed in r1583, but make sure we're testing this from now on)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1637 7f0cb862-5218-0410-a997-914c9d46530a