GDesktopAppInfo: simplify how defaults work

GDesktopAppInfo: simplify how defaults work

Previously, we took the default application for a particular mimetype
from the system and copied it into the user's configuration as the
default there.

Instead of doing that we leave the user's default unset, and at time of
use, if the user has no explicitly-set default value, we use the system
default.

This avoids complicated situations where inappropriate applications were
being set as the default in the user's configuration.

https://bugzilla.gnome.org/show_bug.cgi?id=658188