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)

Finished cleanup of outdated SP_OBJECT_PARENT C macro.

From trunk

remove usereference vestiges, change the signature of user_unlink (not used yet)

LPE STACKING! (many thanks to the french students who made this.)

remaining g++ 4.3 compilation fix-ups

warning cleanup

trivial: live_effects/**: svn propset svn:eol-style native *.h *.cpp.

Fix LPE: * add default values for parameters * remove text for LPE_INVALID * update example for skel strokes * clean a bit of the code

Commit LivePathEffect branch to trunk! (disabled extension/internal/bitmap/*.* in build.xml to fix compilation)