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/texteditor/texteditor.h

@@ -666,8 +666,6 @@ public:
     void setParenthesesMatchingEnabled(bool on);
     void setCodeFoldingSupported(bool on);
 
-    Core::IEditor *createEditor() override;
-
 private:
     friend class BaseTextEditor;
     friend class PlainTextEditorFactory;