Add helper methods for callback functions

[jebej]

Would you be interested in having helper methods to set the callback functions which automatically create the function pointer with @cfunction ? Since the signature is fixed by the library we can just assume that the user is passing a valid method.

Eg. we could define:

function mfb_set_keyboard_callback(window, keyboard::Function)
    keyboard_c = @cfunction(keyboard, Cvoid, (Ptr{Cvoid}, mfb_key, mfb_key_mod, Bool))
    mfb_set_keyboard_callback(window, keyboard_c)
end

Let me know if that would be good to have, and if the naming, or something else would need to be changed

[aviks]

Yeah, that might be a good idea!

[jebej]

Any input on the api? Should I just add methods to the various mfb_set_*_callback functions?

[jebej]

I had to add $ to the macro call since the function pointer must be created at runtime. I am not sure whether that's a drawback or not.