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/trafficgraphwidget.cpp

@@ -139,10 +139,10 @@ void TrafficGraphWidget::updateRates()
     }
 
     float tmax = 0.0f;
-    foreach(float f, vSamplesIn) {
+    Q_FOREACH(float f, vSamplesIn) {
         if(f > tmax) tmax = f;
     }
-    foreach(float f, vSamplesOut) {
+    Q_FOREACH(float f, vSamplesOut) {
         if(f > tmax) tmax = f;
     }
     fMax = tmax;