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/addressbookpage.h

@@ -45,7 +45,7 @@ public:
     void setModel(AddressTableModel *model);
     const QString &getReturnValue() const { return returnValue; }
 
-public slots:
+public Q_SLOTS:
     void done(int retval);
 
 private:
@@ -59,7 +59,7 @@ private:
     QAction *deleteAction; // to be able to explicitly disable it
     QString newAddressToSelect;
 
-private slots:
+private Q_SLOTS:
     /** Delete currently selected address entry */
     void on_deleteAddress_clicked();
     /** Create a new address for receiving coins and / or add a new address book entry */
@@ -80,7 +80,7 @@ private slots:
     /** New entry/entries were added to address table */
     void selectNewAddress(const QModelIndex &parent, int begin, int /*end*/);
 
-signals:
+Q_SIGNALS:
     void sendCoins(QString addr);
 };