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.

Conflicts:
	src/qt/addressbookpage.cpp
	src/qt/peertablemodel.cpp
	src/qt/receivecoinsdialog.cpp
	src/qt/rpcconsole.cpp

Rebased-From: d29ec6c2301e593d577126d1ca85b93307b32bf1
Github-Pull: #6433

src/qt/intro.h

@@ -43,14 +43,14 @@ public:
      */
     static QString getDefaultDataDirectory();
 
-signals:
+Q_SIGNALS:
     void requestCheck();
     void stopThread();
 
-public slots:
+public Q_SLOTS:
     void setStatus(int status, const QString &message, quint64 bytesAvailable);
 
-private slots:
+private Q_SLOTS:
     void on_dataDirectory_textChanged(const QString &arg1);
     void on_ellipsisButton_clicked();
     void on_dataDirDefault_clicked();