[EAK-643] Add method(s) to ListHelper allowing to modify the list of items

[smiakchilo]

As a developer with EToolbox Authoring Kit, I use ListHelper ro read contents of EToolbox Lists. I also have the possibility to create a list in code. The limitation is that I can only create a list as a whole. I would enjoy the possibility to:

• insert item(-s) at a particular position;
• remove particular item(s);
• modify items' properties;
• reorder items
  -- all through predefined utility methods that make it not necessary to follow the internal buildup of an EToolbox List in client code.