XDG Default Applications specification proposal

[Vladimir Kudrya]

Hi.

Terminal is a bit of a special case with additional launch args and does 
not fit well into MIME handling. I already made a proposal with a 
demonstrator some time ago on this list. Started somewhere around this 
https://lists.freedesktop.org/archives/xdg/2017-May/013909.html and 
resulted into this: https://github.com/Vladimir-csp/xdg-terminal-exec