Replace Util::ptr_shared with std::shared_ptr

We currently define our own shared pointer class in src/util/share.h

Since C++11 has now been adopted in lp:inkscape, we should replace the usage of ptr_shared with the shared_ptr class, as defined in the standard <memory> C++ header, and delete our own implementation.

The standard implementation is likely to be more robust than ours and more recognisable to new contributors, so it makes sense to migrate.

This is feature is very widely adopted in current C++ implementations, so it should be safe for us to use right away.