This makes D-Bus proxy signal registration more flexible, more dynamic, and less error-prone since no `finishRegistration()` call is needed. A proxy can register to a signal at any time during its lifetime, and can unregister freely by simply destroying the associated slot.
This improves the D-Bus object API registration/unregistration by making it more flexible, more dynamic, closer to sd-bus API design but still on high abstraction level, and -- most importantly -- less error-prone since no `finishRegistration()` call is needed anymore.
* feat: add async property get/set convenience support classes
* feat: add no-reply and async overloads to Properties_proxy
* feat: add convenience functions for GetAll functionality
* test: add tests for new functionality
* add codegen IDL support and documentation
