Support for projects using QT_NO_KEYWORDS

[pedrolcl]

https://doc.qt.io/qt-6/signalsandslots.html#signals-and-slots-in-qt-based-libraries

The public API of Qt-based libraries should use the macros Q_SIGNALS and Q_SLOTS instead of signals and slots. Otherwise it is hard to use such a library in a project that defines QT_NO_KEYWORDS.

To enforce this restriction, the library creator may set the preprocessor define QT_NO_SIGNALS_SLOTS_KEYWORDS when building the library.