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/paymentserver.h

@@ -98,7 +98,7 @@ public:
     // Verify the payment request amount is valid
     static bool verifyAmount(const CAmount& requestAmount);
 
-signals:
+Q_SIGNALS:
     // Fired when a valid payment request is received
     void receivedPaymentRequest(SendCoinsRecipient);
 
@@ -108,7 +108,7 @@ signals:
     // Fired when a message should be reported to the user
     void message(const QString &title, const QString &message, unsigned int style);
 
-public slots:
+public Q_SLOTS:
     // Signal this when the main window's UI is ready
     // to display payment requests to the user
     void uiReady();
@@ -119,7 +119,7 @@ public slots:
     // Handle an incoming URI, URI with local file scheme or file
     void handleURIOrFile(const QString& s);
 
-private slots:
+private Q_SLOTS:
     void handleURIConnection();
     void netRequestFinished(QNetworkReply*);
     void reportSslErrors(QNetworkReply*, const QList<QSslError> &);