Consistency of ellipses across the UI

Consistency of ellipses across the UI

Use the function support:truncateWithEllipsis() to shorten a docstring with
... at the end. Actually we use U+2026 HORIZONTAL ELLIPSIS instead of "..." when
automatically shortening strings. This is to be consistent with Qt's own
truncation and is much nicer on the screen.

This includes the bugs #9575 and #9572 regarding broken text elision in the
outliner.

Known issues (non-regressions):

* TocBackend::updateItem() should be rewritten to update all TOCs. (#8386)

* "..." should be replaced with … everywhere else on the interface (including
  translation strings).

* We should prefer to rely on QFontMetrics::elidedText() to truncate strings
  with an ellipsis whenever possible, or an equivalent for the buffer view
  dependent on the font metrics. See the warning in src/support/lstrings.h.