trackobject: Properly rename get/set_child_property

trackobject: Properly rename get/set_child_property

We used to have a ges_track_object_get/set_child_property that was in
fact letting user set/get various properties at once, rename it to
get/set_properties, and implement:

API:
    ges_track_object_get_child_property (GESTrackObject *object, const gchar
            *property_name, GValue * value);
    ges_track_object_set_child_property (GESTrackObject *object, const gchar
            *property_name, GValue * value);