QtSupport: Fix partial delete of ExampleItem objects

If we delete items via the base class destructor, it needs to be
virtual.

Change-Id: Id5deaa3f327d881b93852b5cbf1cf2940bb333d0
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/welcomepagehelper.h

@@ -88,6 +88,7 @@ private:
 class CORE_EXPORT ListItem
 {
 public:
+    virtual ~ListItem() {}
     QString name;
     QString description;
     QString imageUrl;

src/plugins/qtsupport/exampleslistmodel.h

@@ -103,6 +103,8 @@ enum InstructionalType
 class ExampleItem : public Core::ListItem
 {
 public:
+    ~ExampleItem() override {}
+
     QString projectPath;
     QString docUrl;
     QStringList filesToOpen;