Update region_ops.txt to match current behaviour, and fix a couple of comments.

Update region_ops.txt to better describe the current behaviour of
Editor::get_regions_from_selection_and_edit_point(), and fix a couple of
comments that still referred to the no-longer-existing 'edit' property of
route groups.
vtl: video-monitor letterbox & orig-zoom to menu
vtl: audio-export progress-bar tweaks

use the same concept as session > export > audio;
hopefully this fixes jack-disconnects during export.
vtl: re-order view menu entries
vtl: A/V sync lock on import

When extracting and importing audio from a video,
mark the audio region's position as locked to the
video.

While locked, the audio region can only be moved
by moving the video with a granularity of one Timecode
frame.