on the timeline; adding a new change at the same time as an existing one
will replace the existing one. Should prevent #769 from happening.
Some cleanups and tweaks to tempo / meter dialogues. Desensitize Remove
menu option for those changes that can't be removed.
git-svn-id: svn://localhost/ardour2/branches/3.0@7045 d708f5d6-7413-0410-9779-e7cbd77b26cf