TextEditor: Use more direct approach for fallback IEditor construction

There are still a few places where the EditorWidget is the primary
object, or several of them per "visible" IEditor (e.g. Diff).
Provide a means to streamline setup there, too.

Change-Id: I14cfbd68e555ebc539e707032a0e5bef563e0a36
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/diffeditor/diffeditor.cpp

@@ -83,7 +83,6 @@ public slots:
     void setDisplaySettings(const DisplaySettings &ds);
 
 protected:
-    BaseTextEditor *createEditor() { return new BaseTextEditor; }
     void mouseMoveEvent(QMouseEvent *e);
     void mouseReleaseEvent(QMouseEvent *e);
 
@@ -98,7 +97,8 @@ private:
 DescriptionEditorWidget::DescriptionEditorWidget(QWidget *parent)
     : BaseTextEditorWidget(parent)
 {
-    setSimpleTextDocument("DiffEditor.DescriptionEditor");
+    setupFallBackEditor("DiffEditor.DescriptionEditor");
+
     DisplaySettings settings = displaySettings();
     settings.m_textWrapping = false;
     settings.m_displayLineNumbers = false;