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

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

@stefansaraev suggested on irc something like https://github.com/xbmc/xbmc/pull/5326 suggests

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

So when https://github.com/xbmc/xbmc/pull/5326 is merged maybe
this format https://github.com/taxigps/xbmc/commit/c0493be0ec7622840736ef22c29a92d3295e2e36#diff-705f1cb5ddd9d1307893a6cfe1f1f7fcR183 could be used instead throughout

So the list selection would display the layout name and the button label
would display the id, this would be ideal. But that is not yet supported in code.

This is here just to show what could be done to improve this situation
and make it clearer what my comments meant to @tamland

I've added a small comment to the xml to serve as minor document so all future considerations
to following some predefined "standard" sensible and logic.