PR check list:
- [ ] Document your code
- [ ] Add `\since QXmpp 1.X`, `QXMPP_EXPORT`
- [ ] Fix doxygen warnings (see log when building with `-DBUILD_DOCUMENTATION=ON`)
- [ ] Update `doc/doap.xml`
- [ ] Add unit tests
- [ ] Format the code: Run `clang-format -i src/<edited-file(s)> tests/<edited-file(s)>`

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