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/overviewpage.h

@@ -35,11 +35,11 @@ public:
     void setWalletModel(WalletModel *walletModel);
     void showOutOfSyncWarning(bool fShow);
 
-public slots:
+public Q_SLOTS:
     void setBalance(const CAmount& balance, const CAmount& unconfirmedBalance, const CAmount& immatureBalance,
                     const CAmount& watchOnlyBalance, const CAmount& watchUnconfBalance, const CAmount& watchImmatureBalance);
 
-signals:
+Q_SIGNALS:
     void transactionClicked(const QModelIndex &index);
 
 private:
@@ -56,7 +56,7 @@ private:
     TxViewDelegate *txdelegate;
     TransactionFilterProxy *filter;
 
-private slots:
+private Q_SLOTS:
     void updateDisplayUnit();
     void handleTransactionClicked(const QModelIndex &index);
     void updateAlerts(const QString &warnings);