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>