message: Changelog.md: Adds fix-notice.
diff: - Fixes Sixel image rendering when scrolling is needed and ANSI cursor is not on left margin.
  - Fixes Qt-related CLI options that that were largely ignored.
  +- Fixes crash caused by VT sequence PM and SOS (#513).
  - Changes `contour` exit code to reflect the shell's exit code of the last closed window.
  - Adds CLI option `terminal early-exit-threshold SECS` (defaulting to 6) to only report and wait if the process did exit below this threshold seconds.
  - Adds CLI option `terminal dump-state-at-exit` to auto-dump internal state at exit.
  