virtualize the way that AutomationController gets strings to display values, so that...

virtualize the way that AutomationController gets strings to display values, so that we can callback through the owner of an AutomationControl, not just rely on the value from the AutomationControl; make pan automation tracks use this to display more audio-centric values

git-svn-id: svn://localhost/ardour2/branches/3.0@8590 d708f5d6-7413-0410-9779-e7cbd77b26cf