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/unifieddiffeditorwidget.h

@@ -68,7 +68,6 @@ public slots:
 protected:
     void mouseDoubleClickEvent(QMouseEvent *e);
     void contextMenuEvent(QContextMenuEvent *e);
-    TextEditor::BaseTextEditor *createEditor();
     QString lineNumber(int blockNumber) const;
     int lineNumberDigits() const;