server: Make some SimpleContainer methods protected and documented.
server: Make some SimpleContainer methods protected and documented.

Make get_all_children() and is_child_id_unique() protected and
document them as being useful when implementing derived classes,
because they are only used in plugins that derive their own
containers.

GNOME/rygel,GNOME/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,jku/rygel,jku/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched