A lot of header clean-up.

Util::ptr_shared<T> was only used in its <char> specialization, so it is now refactored into a
non-template class. Using it with arbitary classes was dangerous anyway.

(bzr r15034)

(bzr r15027)

(bzr r15025)

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)

(bzr r14245)

(bzr r10067)

(bzr r4052)

(bzr r3227)