Create the session range location as and when the session first gets some content...

Create the session range location as and when the session first gets some content.  Allows both the beginning and end of the range to expand to contain the actual session contents.

git-svn-id: svn://localhost/ardour2/branches/3.0@7087 d708f5d6-7413-0410-9779-e7cbd77b26cf