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)

- try to use more forward declarations for less dependencies on display/curve.h - change _bpath to private member of SPCurve, obtain and set with get_bpath and set_bpath. - added const methods, so protect changes to _bpath in SPCurve

svn propset svn:eol-style native *.[ch] *.cpp (including buildtool.cpp, packaging/macosx/native-gtk/*, src/dom/**).

add linking to other paths to lpe-PathParam