The file format does not longer contain object IDs because references are
not used. This simplifies the file format, leads to shorter files and
supportes merges between model files much better - changes in model are
reflected by minimal changes in the file.

Change-Id: I8c76b7899306eec9a493c6cf61add02111229760
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>