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/addresstablemodel.h

@@ -84,7 +84,7 @@ private:
     /** Notify listeners that data changed. */
     void emitDataChanged(int index);
 
-public slots:
+public Q_SLOTS:
     /* Update address list from core.
      */
     void updateEntry(const QString &address, const QString &label, bool isMine, const QString &purpose, int status);