major, substantive reworking of how we store GUI information (visibility, height...

major, substantive reworking of how we store GUI information (visibility, height) for automation data. old design stored (insufficient) identifying information plus actual data in a GUI-only XML node; new scheme adds GUI data via extra_xml node to each AutomationControl object. reworked public/private methods for showing/hiding TimeAxisView objects; changed labelling of automation tracks to just show the name of the controlled parameter - more info can be viewed in the tooltip for the track headers. NOTE: Session file format ALTERED. No data loss but track visibility may be different than previous ardour3 versions

git-svn-id: svn://localhost/ardour2/branches/3.0@9703 d708f5d6-7413-0410-9779-e7cbd77b26cf