Add new SharedStatefulProperty which manages a shared_ptr to

Add new SharedStatefulProperty which manages a shared_ptr to
some Stateful object, and a subclass to use this for
AutomationList.  SharedStatefulProperty will manage undo / redo
using full copies of the XML state, like MementoCommand,
but does it within the Property undo system.

git-svn-id: svn://localhost/ardour2/branches/3.0@12740 d708f5d6-7413-0410-9779-e7cbd77b26cf