Very many testcases, some GLib tools (resource compiler, etc) and
GApplication were calling g_type_init().
Remove those uses, as they are no longer required.
https://bugzilla.gnome.org/show_bug.cgi?id=686161
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
