debugger: centralize check for changed display entries

Change-Id: I22c58812990925102aeeb80571bb877fd0a2924b
Reviewed-on: http://codereview.qt.nokia.com/3330
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/debugger/watchdata.h

@@ -113,6 +113,7 @@ public:
     quint64    coreAddress() const;
     QByteArray hexAddress()  const;
     QByteArray hexReferencingAddress()  const;
+    bool hasChanged(const WatchData &old) const;
 
 public:
     quint64    id;           // Token for the engine for internal mapping