Make qdoc an officially supported tool for generating QML API documentation

Until now qdoc has only been seen as an internal tool to create the Qt Documentation, and it has been recommended to third parties to create their API documentation with other tools like doxygen.

With the arrival of QML there comes an even stronger requirement to create documentation for QML APIs by third parties. This is because one of the main advantages of QML is the separation of GUI and logic code. Since QML GUI designers don't have to necessarily know C++, it is not feasible to hand them a bunch of header files, which makes proper out-of-code documentation a must. qdoc is the only tool at the moment, that can generate documentation for QML APIs, but since it's not supported for external usage, there is no official way to generate QML API documentation, although it's so much needed.

Since doxygen is not expected to support JavaScript and QML soon, and qdoc already has solved a couple of complex problems regarding QML documentation generation, I suggest to elevate qdoc to an official tool also for externally implemented QML APIs.

At the moment the qdoc documentation is far from complete and some stuff, like the \inherit command, does not work if you don't build the docs alongside with Qt's documentation. Making qdoc an official tool for external usage would give it a higher priority to fix these things.