new approach to tempo/meter: compute and store the entire map (every bar|beat point...

new approach to tempo/meter: compute and store the entire map (every bar|beat point), thus enabling us to use the same computation to set the BBT points AND the metric markers (tempo + meter) on the audio timeline. It is known that snapping to the BBT grid doesn't work correctly right now, but this probably caused by the separate code in TempoMap::round_to_type() and i'll dig into that tomorrow. Note that the Bar|beat point list is evaluated "lazily" - we'll never store more than anyone actually needs to display or know, other than 1 minute's worth starting from frame zero

git-svn-id: svn://localhost/ardour2/branches/3.0@11129 d708f5d6-7413-0410-9779-e7cbd77b26cf