4.17.3

- Update COPYING (!27)
- Bump GObject Introspection minim version to 1.66 (xfconf#8)
- docs: Add missing index of deprecated symbols
- docs: Fix broken links
- docs: Fix "unused declarations" warning
- docs: Update `.gitignore`
- Remove limit on the size of configuration file line (!29)
- Fix documentation of 'xfce_str_is_empty'
- Clean the build system and use the new XDT_VERSION_INIT (!9)
- Silence `-Wanalyzer-null-dereference` warnings
- doc: Fix typo in `xfce_*_append_quoted()`
- xfce-miscutils: Fix `deadcode.DeadStores` warning from `scan-build`
- i18n: Make sure to set locale according to environment (!26)
- Translation Updates:
  Azerbaijani, Chinese (China), Georgian, Greek