put maybe_unused instead of constexpr

[fabien-colonnier]

Modify the default virtual function for key_callback_extra and mouse_button_callback_extra to avoid the unused arguments error in c++14

[fabien-colonnier]

I also checked the other header files to manage consitency in the repo, only BezCurve.h has also a similar pattern with the variable debug_bezcurve but the variable are used even when the parameter is set to false. So I didn't modify the file

[sebjameswml]

Thanks Fabien!