4.15.0 release

- Implement custom test driver
- Use 'gsettings' as a common channel name for the GSetting backend
- Bug #14789 Fix GSettings backend assertion error, when probed but not used
- Enable GSettings backend build by default
- Add Bash completion for xfconf-query
- Bug #15960: Add some missing xfconf_shutdown() in xfconf-query
- Bug #15951: Fix 'xfconf_get_error_quark' introspecting warning
- Remove perl bindings
- Fix vala binding in dist tarball
- Translation Updates: be, da, el, es, he, hr, hu, hye, gl, nl,
  sl, sq, zh_CN, zh_HK, zh_TW
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