DiffEditor: Pimpl plugin

The standard pattern, and allows to drop the QObject parent on
the editor factory which gets into the way of de-QObject-ifing
the IEditorFactory hierarchy.

Change-Id: I5c6a50f8075a99592ed4459b417ca8ba7471ae96
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/diffeditor/diffeditorfactory.h

@@ -33,7 +33,7 @@ namespace Internal {
 class DiffEditorFactory : public Core::IEditorFactory
 {
 public:
-    explicit DiffEditorFactory(QObject *parent);
+    DiffEditorFactory();
 };
 
 } // namespace Internal