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/watchutils.cpp

@@ -29,6 +29,8 @@
 #include "watchutils.h"
 #include "watchdata.h"
 
+#include <utils/qtcfallthrough.h>
+
 #include <QDebug>
 
 #include <string.h>
@@ -231,8 +233,10 @@ QString formatToolTipAddress(quint64 a)
         switch (rc.size()) {
         case 16:
             rc.insert(12, colon);
+            Q_FALLTHROUGH();
         case 12:
             rc.insert(8, colon);
+            Q_FALLTHROUGH();
         case 8:
             rc.insert(4, colon);
         }