Move IEditor::isTemporary to IDocument

It's a property of the document implementation, not of the specific
editor instance working on it.

Change-Id: I5c3dd054e21b646e2d94b891916a096d045923f8
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/diffeditor/diffeditor.h

@@ -64,7 +64,6 @@ public:
     bool open(QString *errorString, const QString &fileName, const QString &realFileName);
     Core::IDocument *document();
     Core::Id id() const;
-    bool isTemporary() const { return true; }
     DiffEditorWidget *editorWidget() const { return m_editorWidget; }
 
     QWidget *toolBar();