Add compatibility with QT_NO_KEYWORDS macro

[dbermond]

The Qt documentation recommends not using Qt keywords in the public API of Qt libraries, specially the signals and slots Qt keywords.

Without this, client applications that are compiled with the QT_NO_KEYWORDS macro (or that are compiled in conjunction with other libraries that also defines the same Qt keywords) will fail to build.

The QT_NO_SIGNALS_SLOTS_KEYWORDS macro assures that the library is not using signals and slots Qt keywords, since it will fail to build when they are used. Useful for code sanity and for avoiding future pull requests that may try to use these Qt keywords.