Contiainer should track changes to its content
Currently there is no way to detect, if the content of a container has changed without manually iterating over all contained references. This is unfortunate for the container UI and a real problem for weight calculations.
(RM-217 from redmine: created on 2012-03-19 by Marc Zinnschlag, , closed on 2012-03-22 by Marc Zinnschlag)