XDG Default Applications specification proposal

[Thayne]

A few months ago I put together a draft specification for a standard
mechanism for specifying default applications for things like terminal
emulators, calculators, etc.  that don't fit aren't necessarily opened
with a file with a MIME type:

https://gist.github.com/tmccombs/1642aacd4f6f8078ac0a718b72a65699

In particular, this specifies which terminal application should be
used to launch applications from desktop entries with Terminal=true.

My main motivation for writing this is
https://gitlab.gnome.org/GNOME/glib/-/issues/338 .

I would appreciate any feedback.

- Thayne