Extract Core::TextFile from TextEditor::BaseTextDocument.

Use it in BaseTextEditor and Designer to preserve CRLF
of the files.

Task-number: QTCREATORBUG-5901
Change-Id: I7599ce78649a3b09f2e5118d02f8cbf3db27ed31
Reviewed-on: http://codereview.qt.nokia.com/3591
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/plugins/texteditor/basetexteditor.cpp

@@ -5615,7 +5615,8 @@ void BaseTextEditorWidget::setTextCodec(QTextCodec *codec)
 
 QTextCodec *BaseTextEditorWidget::textCodec() const
 {
-    return baseTextDocument()->codec();
+    // TODO: Fix all QTextCodec usages to be const *.
+    return const_cast<QTextCodec *>(baseTextDocument()->codec());
 }
 
 void BaseTextEditorWidget::setReadOnly(bool b)