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/sendcoinsentry.h

@@ -45,15 +45,15 @@ public:
 
     void setFocus();
 
-public slots:
+public Q_SLOTS:
     void clear();
 
-signals:
+Q_SIGNALS:
     void removeEntry(SendCoinsEntry *entry);
     void payAmountChanged();
     void subtractFeeFromAmountChanged();
 
-private slots:
+private Q_SLOTS:
     void deleteClicked();
     void on_payTo_textChanged(const QString &address);
     void on_addressBookButton_clicked();