A position updater is responsible for adapting document positions. When
installed on a document, the position updater updates the document's
positions to changes applied to this document. Document updaters can be
selective, i.e. they might only update positions of a certain category.
Position updaters are of primary importance for the definition of the
semantics of positions.
Adapts positions to the change specified by the document event.
It is ensured that the document's partitioning has been adapted to
this document change and that all the position updaters which have
a smaller index in the document's position updater list have been called.
event - the document event describing the document change