X11 support for mfb_set_char_input_callback

[enjector]

Char input events weren't being generated under X11. This extracts the character code from the event and calls the callback specified by mfb_set_char_input_callback(...)

[Darky-Lucera]

Finally, I figure out how it works and I have changed the code to be able to get Unicode characters like ñ, ç, á, è, î, ... but for now it only works correctly on MacOS X and Windows. There is some support for X11 but it seems that X11 libs are older than Unicode, so it is a bit difficult to do it.

I tried this: http://unicode.org/mail-arch/unicode-ml/Archives-Old/UML015/0422.html but it does not work with compound characters

For Linux, I guess that we need to invest more time in this: https://xkbcommon.org/
xkb_state_key_get_utf32() seems to be the way to do it