Refactor VisB Items to detect duplicate entries

Currently VisBItems is an ArrayList and duplicate entries for id/attribute are not detected.

It would be nice to

• remove overwritten id/attribute entries (from included .json files)

• create a warning when an item from the same .json file is overwritten

Currently new items are added in VisBFileHandler.java using visBItems.add(new VisBItem(id, attribute, value));
One should probably use a hash map to efficiently detect multiple entries.

The same holds for VisBEvents.