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.cpp

@@ -142,7 +142,7 @@ OverviewPage::OverviewPage(QWidget *parent) :
 void OverviewPage::handleTransactionClicked(const QModelIndex &index)
 {
     if(filter)
-        emit transactionClicked(filter->mapToSource(index));
+        Q_EMIT transactionClicked(filter->mapToSource(index));
 }
 
 OverviewPage::~OverviewPage()