0.21.1

• Fix error disconnecting text view's line length margin handler
  (AttributeError: 'TextView' object has no attribute 'gaupol_ruler_handler_id')
• Fix atomic file writing in weird cases where the subtitle file to
  be written and its backup in the same directory would be on
  different filesystems (Florian Léger, Osmo Salomaa)
• Fix speed issues updating subtitle list selection (e.g. when doing
  a search-and-replace-all with a alot of matches)
• Speed up action sensitivity updates