qt: define QT_NO_KEYWORDS

QT_NO_KEYWORDS prevents Qt from defining the `foreach`, `signals`,
`slots` and `emit` macros.

Avoid overlap between Qt macros and boost - for example #undef hackiness
in #6421.

src/qt/clientmodel.h

@@ -82,7 +82,7 @@ private:
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();
 
-signals:
+Q_SIGNALS:
     void numConnectionsChanged(int count);
     void numBlocksChanged(int count, const QDateTime& blockDate);
     void alertsChanged(const QString &warnings);
@@ -94,7 +94,7 @@ signals:
     // Show progress dialog e.g. for verifychain
     void showProgress(const QString &title, int nProgress);
 
-public slots:
+public Q_SLOTS:
     void updateTimer();
     void updateNumConnections(int numConnections);
     void updateAlert(const QString &hash, int status);