[Gzz] 3rd (Benja)

[Benja Fallenstein]

- Moved stuff to gzz.slices.
- Created more generic gzz.util.Version interface for 
gzz.slices.SliceVersion to implement. With that generic interface, we 
can write a generic Mediaserver serializer that can do versioning for 
any gzz.util.Version implementation.
- Made Span serialization work. I had to make ScrollBlock an interface 
for this (see commit message). All scroll Spans can now potentially be 
serialized; a Mediaserver.Id is stored instead of the ScrollBlock 
reference. Spans can only be serialized and deserialized using 
MediaInputStreams and MediaOutputStreams instead of Object*Streams; the 
Media*Streams provide a Mediaserver.
- Some fixes.

-b.