This commit also changes (maintaining compatibility) the way
user-specified default applications are stored (as in, those for which
g_app_info_set_as_default_for_type() has been called.

We now store the default application for a content type in a new group
in the mimeapps.list keyfile, and "Added Associations" tracks only the
applications that have been added by the user, following a
most-recently-used first order.

This is useful in GtkAppChooser-like widgets to pre-select the last used
application when constructing a widget.

https://bugzilla.gnome.org/show_bug.cgi?id=636311