Add Q_FALLTHROUGH for Qt < 5.8

... and make use of it.
With gcc 7, the new option -Wimplicit-fallthrough is introduced and
added to the -Wextra set, triggering dozens of warnings in our sources.
Therefore, we annotate all obviously intended fall-throughs. The ones
that are still left are unclear and need to be checked by the respective
maintainer.

Change-Id: I44ead33cd42a4b41c28ee5fcb5a31db272710bbc
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/qml/qmlengine.cpp

@@ -60,6 +60,7 @@
 #include <utils/treemodel.h>
 #include <utils/basetreeview.h>
 #include <utils/qtcassert.h>
+#include <utils/qtcfallthrough.h>
 
 #include <QDebug>
 #include <QDir>
@@ -505,7 +506,7 @@ void QmlEngine::errorMessageBoxFinished(int result)
     }
     case QMessageBox::Help: {
         HelpManager::handleHelpRequest("qthelp://org.qt-project.qtcreator/doc/creator-debugging-qml.html");
-        // fall through
+        Q_FALLTHROUGH();
     }
     default:
         if (state() == InferiorRunOk) {