Some of them need to be ordered in STL containers, and thus need
a special comparable wrapper for boost::shared_ptr, defined in
comparable_shared_ptr.h. This also alleviates the typedef hell
present earlier in some export classes :)
Making the timespan pointer comparable should fix bug #4093
git-svn-id: svn://localhost/ardour2/branches/3.0@9702 d708f5d6-7413-0410-9779-e7cbd77b26cf