DiffEditor: Make it possible to paste snippets.

Add property "plainText" to DiffEditorDocument which
is queried as last resort by the code paster.

Task-number: QTCREATORBUG-12546
Task-number: QTCREATORBUG-9669
Change-Id: Iea9cd0d296bcc8e8a7300072c1a4a9068c2ab102
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/diffeditor/diffeditordocument.cpp

@@ -155,4 +155,9 @@ QString DiffEditorDocument::suggestedFileName() const
     return result + QStringLiteral(".patch");
 }
 
+QString DiffEditorDocument::plainText() const
+{
+    return m_controller->contents();
+}
+
 } // namespace DiffEditor