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/bitcoinamountfield.h

@@ -56,7 +56,7 @@ public:
     */
     QWidget *setupTabChain(QWidget *prev);
 
-signals:
+Q_SIGNALS:
     void valueChanged();
 
 protected:
@@ -67,7 +67,7 @@ private:
     AmountSpinBox *amount;
     QValueComboBox *unit;
 
-private slots:
+private Q_SLOTS:
     void unitChanged(int idx);
 
 };