Move property merging to free functions to allow testing

Nesting the determination of common properties inside the provider, which can be updated only through the filefetchjob, makes unit testing impossible.

The first commit moves the code to separate free functions. The only actual change is providing the former QVariantMap m_data member as first parameter by reference.

The second commit adds unit tests and exercises the functions.

The third commit finally removes the no longer private methods.