Typedef all globally used export smart pointer types in one file.

Typedef all globally used export smart pointer types in one file.
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