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()
New semantics for ExtrusionLoop objects. Early processing of perimeter overhangs for paralellizing such work and making G-code export lighter. Lots of refactoring. This should fix a number of minor bugs, including reversals of perimeter overhangs.
Merge branch 'sapir-modelcpp'
Several minor fixes to Model
Replace to_SV_ref() and to_SV_clone_ref() with templated glue functions
Remove new/delete keywords from ClipperUtils.cpp
Refactoring to Model API for making it stricter and safer
Merge pull request #19 from alexrj/master

Merge from master