Smartpointer refactor

Replace std::auto_ptr, boost::scoped_ptr and boost::shared_ptr with C++11 smart pointer.

Make Inkscape::Util::ptr_shared a non-template class, as only its char specialization is ever used. Using it with arbitary Objects is dangerous anyway. Util::ptr_shared (and the code using it) relies on the fact that _obj is always allocated by the garbage collector (or never freed before program termination), which is not guaranteed by any function in share.h except for share_string.

Note that there are still boost::shared_ptr in 2geom.