Fix callbacks in GLWin not working in objects that aren't subclasses of GLWin

[dkrautha]

Mainly to allow making keybinds in AudioPlayer and VideoPlayer

[ahuston-0]

@dkrautha implemented a promising alternative to this called WantsInput. It currently just intercepts key callbacks, but could be implemented to work with other types of callbacks as well. Both this and WantsInput will need to be finished and compared to determine which is the better solution, but WantsInput may end up being the more efficient of the two (as it doesn't seem to require any function pointers or std::functions, and instead takes a pointer to the class).