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.cpp

@@ -450,5 +450,5 @@ int AddressTableModel::lookupAddress(const QString &address) const
 
 void AddressTableModel::emitDataChanged(int idx)
 {
-    emit dataChanged(index(idx, 0, QModelIndex()), index(idx, columns.length()-1, QModelIndex()));
+    Q_EMIT dataChanged(index(idx, 0, QModelIndex()), index(idx, columns.length()-1, QModelIndex()));
 }