Translate Model class' storage to C++.

Some code copied from xs-model branch.

Also:
* Generate ::Ref classes programatically.
* Add separate __REGISTER_CLASS macro
    (for use where forward declaration won't work, i.e. typedefs)
Removed StringMap
Use Pointf for origin_translation and pass const refs whenever possible
Don't transfer ownership of Model to Print::Object
Fix clear_objects() and clear_volumes()
Merge branch 'sapir-modelcpp'