Don't do menu operations while menus are open

[3b]

per GLUT spec (and enforced by freeglut):

It is illegal to create or destroy menus, or change, add, or remove menu items while a menu (and any cascaded sub-menus) are in use (that is, popped up).

To avoid problems, we need to check for open menus and defer any modifications until they close. Easiest way to trigger a problem is to open a menu and then close the window with a key (closing from the menu or using window decorations usually closes the menu early enough to be safe). Similarly, modifying menus from a keypress or other callback while menus are open is a problem. Possibly opening new windows with menus while menus are open would also break on some configurations (when I tried on windows, the menu closes when the new window is created, but I'm not sure if that is guaranteed to work or if it just happened to work because of focus changes or something).

Not completely trivial to get general deferred operations right though, since windows could possibly have been created and destroyed while a menu was open, and we need to avoid creating/destroying menus for that window or worse creating them and attaching them to a new window with the same id.