VolumeContentBuilder allows invalid sequences of operation

[GoogleCodeExporter]

The VolumeContentBuilder currently allows invalid sequences of operation, such 
as
newSequence(null);
newOnTocStart();

Doing so could throw an IllegalStateException, but it is really up to the 
implementation to do so. This causes unnecessary uncertainty which can be 
avoided by moving the state dependent methods to separate interfaces. E.g. 
newSequence(null) could return a FormatterCore for that sequence.

And newTocSequence(null) could return another interface containing the methods 
needed e.g. newOnTocStart() (which in turn could return FormatterCore);

Original issue reported on code.google.com by joel.hak...@mtm.se on 21 May 2014 at 7:07