change Session::convert_to_frames_at() to Session::convert_to_frames() to reflect...

change Session::convert_to_frames_at() to Session::convert_to_frames() to reflect the fact that its argument is a position, not a duration; add Session::any_duration_to_frames(), which converts AnyTime representing a duration to frames; alter callers to use the right one of the two previously mentioned methods

git-svn-id: svn://localhost/ardour2/branches/3.0@8386 d708f5d6-7413-0410-9779-e7cbd77b26cf