gio: fixing duplicate icons and review comments.
gio: fixing duplicate icons and review comments.

The basic test for duplicate icon names was not working fine when taking
into account fallbacks and icon style mix. This fixes it.

Also resolve the few review comments, i.e.: comment explaining
g_themed_icon_update_names() and the priority order applied to icons;
and using guint for 'i'.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib