Previously, the PT import would place regions on tracks
numbered from 0, therefore landing on unrelated tracks
when pre-existing tracks were present in the session.
This change skips all existing ardour tracks and imports
the PT regions onto the correctly named, appended tracks.
If any tracks exist with the same names as the ones in the import,
the new tracks get an incremented number at the end of them and
the imported regions are placed on the new track, as expected.
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