Provide alternatives to SIGNAL() and SLOT() macros when QT_NO_KEYWORDS is defined

If QT_NO_KEYWORDS is defined, "signals", "slots" and "emit" are not defined, thus increasing the interoperability with other toolkits by avoiding some common name clashes. However, the SIGNAL() and SLOT() macros are still defined, which may cause name clashes still.

It would be nice if these macros where also not defined when QT_NO_KEYWORDS is defined.