[keyboard layouts] 3 digit ISO 639-2 language + layout

Would superseed https://github.com/xbmc/xbmc/pull/5472
and agreeing that the full language name + layout becomes a longwinded
string to have in the button on OSK

<layout name="English ABC" id=eng_abc>
<layout name="English QWERTY" id=eng_qwerty>

So the list selection would display the layout name and the button label
would display the id, this would be ideal.

This is here just to show what could be done to improve this situation
and make it clearer.

@tamland