Fix for circular references detection in almost all cases, fixing https://bugs.launchpad.net/inkscape/+bug/167247 and a few of its duplicates. This fix is aimed at preventing any sort of circular references with the URIReference::_acceptObject method, checking the absence of loops in the reference+child tree. There can be some performance improvements done if we add a pointer from cloned sub-objects to their origin sub-object. The remaining cases that are not fixed can involve non-trivial loops using one or more "url()" stylesheet references. Being able to take them into account would require a non-obvious style.cpp refactoring making use of URIReference for this kind of reference (and not handling manually the signals in the styling code, which would probably be a good thing to do anyway)

create SPObject factory

Update to experimental r13565

Add selection sets

Start cleanup for merge into trunk

Added a few swatch related functions (does not compile)