Teach qdoc to pull out deprecation messages from QT_DEPRECATED_X

It would be great if qdoc could pull out the deprecation messages from QT_DEPRECATED_X, e.g.:

 QT_DEPRECATED_X("Use QGuiApplication::primaryScreen()") int primaryScreen() const;

That way the message wouldn't need to be duplicated in the documentation for \obsolete as well.

See e.g. the duplication in https://codereview.qt-project.org/#/c/205020/

In general, we use QT_DEPRECATED too much, and should prefer QT_DEPRECATED_X, so that the compiler will emit a deprecation warning with an explanation of what to use instead. One good way to get to more QT_DEPRECATED_X use, would be if qdoc would then automatically do the doc parts for you.

On clang, QT_DEPRECATED expands to_attribute_ ((_deprecated)) and QT_DEPRECATED_X expands to __attribute_ ((_deprecated_(text))), which should be possible to pull out from clang-qdoc.