Core: Fix memory leak

Detected by memcheck

Change-Id: Ic6e1ae6735ebea252858682e1a269f86166a9aa4
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/designmode.cpp

@@ -56,6 +56,7 @@ class DesignModePrivate
 {
 public:
     DesignModePrivate();
+    ~DesignModePrivate();
 
 public:
     QPointer<IEditor> m_currentEditor;
@@ -72,6 +73,11 @@ DesignModePrivate::DesignModePrivate()
       m_stackWidget(new QStackedWidget)
 {}
 
+DesignModePrivate::~DesignModePrivate()
+{
+    delete m_stackWidget;
+}
+
 DesignMode::DesignMode()
     : d(new DesignModePrivate)
 {