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/qmlprofiler/qmlprofilertool.cpp

@@ -69,6 +69,8 @@
 
 #include <qtsupport/qtkitinformation.h>
 
+#include <utils/qtcfallthrough.h>
+
 #include <QApplication>
 #include <QDockWidget>
 #include <QFileDialog>
@@ -387,6 +389,7 @@ void QmlProfilerTool::finalizeRunControl(QmlProfilerRunner *runWorker)
             case QMessageBox::Help:
                 HelpManager::handleHelpRequest(
                             "qthelp://org.qt-project.qtcreator/doc/creator-debugging-qml.html");
+                Q_FALLTHROUGH();
             case QMessageBox::Cancel:
                 // The actual error message has already been logged.
                 logState(tr("Failed to connect."));