use filechooser widget in export dialog, selected files set format combos, hide progr...

use filechooser widget in export dialog, selected files set format combos, hide progress bar until use in export dialog, speed up 'separate regions in range' operation on larger sessions, ruler scale now calculated separately to mark generation, fix for non-stacked layering regression, try not to generate 'buried' crossfades, use playlist->freeze() to speed up copying/moving regions on large playlists (not done for undo), width dependent items now reset on regionview init, get rid of jack_port_ensure_monitor check, remove audiosourse _length (only source has a length.. i think), make overlapend differ to overlapexternal where start points coincide.

git-svn-id: svn://localhost/ardour2/trunk@2576 d708f5d6-7413-0410-9779-e7cbd77b26cf