menu changes based on patch by Jason Wilkins:

menu changes based on patch by Jason Wilkins:
- when menu window is shown, the menu's window no longer becomes the active window on windows (we thus don't steal focus from the active window).
- when menu is active and mouse is pressed outside the active menu and if the pressed button is bound to a menu, the old menu is now closed while a new one immediately appears
- menus now hide before their callback is executed, so they don't linger around if callback execution takes a while

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1583 7f0cb862-5218-0410-a997-914c9d46530a