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/vcsbase/vcsbaseclientsettings.cpp

@@ -29,6 +29,7 @@
 #include <utils/fileutils.h>
 #include <utils/hostosinfo.h>
 #include <utils/qtcassert.h>
+#include <utils/qtcfallthrough.h>
 
 #include <QSettings>
 #include <QVariant>
@@ -56,6 +57,7 @@ public:
         switch (v.type()) {
         case QVariant::UInt:
             m_type = QVariant::Int;
+            Q_FALLTHROUGH();
         case QVariant::Int:
             m_comp.intValue = v.toInt();
             break;