General:
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* buildsystem - Add co-installability of Qt5 and Qt6 headers back.
  Installs Qt6 headers into their own subdirectory so client code still works, but can be co-installed with Qt5 again.

Client-side:
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* Add KDSoapClientInterface::setMessageAddressingProperties() so that WS-Addressing support can be used with WSDL-generated services (issue #254)
* Don't require a SOAP action in order to write addressing properties (also issue #254)

Server-side:
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*

WSDL parser / code generator changes, applying to both client and server side:
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* Improve -import-path support by using the import path in more places in the code