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/test/paymentservertests.cpp

@@ -195,7 +195,7 @@ void PaymentServerTests::paymentServerTests()
     QVERIFY(r.paymentRequest.IsInitialized());
     // Extract address and amount from the request
     QList<std::pair<CScript, CAmount> > sendingTos = r.paymentRequest.getPayTo();
-    foreach (const PAIRTYPE(CScript, CAmount)& sendingTo, sendingTos) {
+    Q_FOREACH (const PAIRTYPE(CScript, CAmount)& sendingTo, sendingTos) {
         CTxDestination dest;
         if (ExtractDestination(sendingTo.first, dest))
             QCOMPARE(PaymentServer::verifyAmount(sendingTo.second), false);