Fixed 'Bug 14530 - GUI Thread hangs a while while xml serialization'

> Xml Serialization is likely the >slowest< way to serialize data and shouldn't be used.
> (btw. nrefactory contains an optimized fast binary serializer - that should be used instead)

I changed the serialization method to the ICSharpCode.NRefactory.Utils.FastSerializer.
It stays backward compatible with the previous implementation (xml serialization) when there is a need for this.