Make GraphRepository persist collections

The GraphRepository can persist GraphContainers.
The current implementations enforces, that each property value is of the following types

• primitve type (e.g. short, byte, boolean)
• string
• enum
• Class

However, if the property value is a collection of the above, persisting fails.

The current behaviour should be changed to allow besides the actually provided property types a collection of those types. The value must be serialized then, maybe as JSON.

If JSON is used as serialization, that could become a supported type as well.