Add fallthrough comments and remove -Wimplicit-fallthrough=0

Signed-off-by: Jonas Kvinge <jonas@jkvinge.net>

src/context/contextview.cpp

@@ -514,6 +514,7 @@ bool ContextView::eventFilter(QObject *object, QEvent *event) {
     case QEvent::Paint:{
       handlePaintEvent(object, event);
     }
+    // fallthrough
     default:{
       return QObject::eventFilter(object, event);
     }