std::function for GLUI callbacks

[nigels-com]

This change replaces GLUI_CB and the associated IDs and pointers with simply std::function<void(void)>. This allows for using C++ lambdas for GLUI callbacks, including std::bind.

For example:

new GLUI_Button( glui, "Quit", []() { printf("Exit\n"); exit(0); } );

[djwebb]

For a c++ idiot like me, could you explain how the callbacks work here - for example at lines 243 and 244 of the original code how does "& {printf etc." link to the callback routine.
Also won't these changes require a reqrite of the manual?
David.