Object Versioning: spec update: new Proxy(..)
Object Versioning: always define __protoProxy as not enumerable..

otherwise the __protoProxy property interferes, for example, with
serialization
Object Versioning: XMLSerializer also needs to be wrapped..

.. used for serialization of worlds ;)
Object Versioning: refactored JSON.prettyPrint, handles proxies
Object Versioning: JSON.stringify can't handle proxies, so unwrap
arguments recursively...

otherwise all proxies are printed as objects, e.g. also proxies for
arrays..
Object Versioning: patch for Strings.print to handle proxies
Object Versioning: lively.undo and .redo redraw all morphs

lively.versions.ObjectVersioning.undo and .redo take a callback, which
in the case of lively.undo and .redo is redrawing all morphs to make
changes between versions visible immediately
Object Versioning: neither copy Global nor DOM stuff for versions
Object Versioning: improved copy-on-write routine