Replaced EMapPropertyHolder and related constructs with an extended
revision of IPropertyHolder and MapPropertyHolder:
 - Added getSerializedProperties() to IPropertyHolder and extended
MapPropteryHolder to provide an implementation for it.
 - Let KGraphElement and KGraphData extend IPropertyHolder, using
MapPropertyHolder within the implementation.
- Changed data of KGraphElement to be EMF-unspecific (Java Object
instead of EObject).