Core: Make IEditorFactory::createEditor use a function object

Also, replace or remove unneeded Q_OBJECTs, and make base
setters and adders protected.

Change-Id: I212257ef53984d8852dc8c478537199fc9483486
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/diffeditor/diffeditorfactory.h

@@ -25,22 +25,15 @@
 
 #pragma once
 
-#include "diffeditor_global.h"
-
 #include <coreplugin/editormanager/ieditorfactory.h>
 
 namespace DiffEditor {
-
 namespace Internal {
 
 class DiffEditorFactory : public Core::IEditorFactory
 {
-    Q_OBJECT
-
 public:
     explicit DiffEditorFactory(QObject *parent);
-
-    Core::IEditor *createEditor() override;
 };
 
 } // namespace Internal