message: Changelog.md cleanup
diff: - Fixes VT sequence `DECFI`.
  - Fixes VT sequence `ICH` (#559).
  +- Fixes VT sequence `OSC 4`'s response.
  - Fixes PTY write race condition.
  - Fixes installation from `.deb` (missing terminfo dependency)
  -- Fixes VT sequence `OSC 4`'s response.
  - Improved VT backend performance (#342).
  - Improved text selection behaviour.
  - Adds new configuration option `mouse_block_selection_modifier`.
  