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/bitcoingui.h

@@ -136,11 +136,11 @@ private:
     /** Disconnect core signals from GUI client */
     void unsubscribeFromCoreSignals();
 
-signals:
+Q_SIGNALS:
     /** Signal raised when a URI was entered or dragged to the GUI */
     void receivedURI(const QString &uri);
 
-public slots:
+public Q_SLOTS:
     /** Set number of connections shown in the UI */
     void setNumConnections(int count);
     /** Set number of blocks and last block date shown in the UI */
@@ -168,7 +168,7 @@ public slots:
     void incomingTransaction(const QString& date, int unit, const CAmount& amount, const QString& type, const QString& address, const QString& label);
 #endif // ENABLE_WALLET
 
-private slots:
+private Q_SLOTS:
 #ifdef ENABLE_WALLET
     /** Switch to overview (home) page */
     void gotoOverviewPage();
@@ -232,7 +232,7 @@ private:
     /** Creates context menu, its actions, and wires up all the relevant signals for mouse events. */
     void createContextMenu();
 
-private slots:
+private Q_SLOTS:
     /** When Display Units are changed on OptionsModel it will refresh the display text of the control on the status bar */
     void updateDisplayUnit(int newUnits);
     /** Tells underlying optionsModel to update its current display unit. */