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/texteditor/snippets/snippeteditor.h

@@ -50,7 +50,6 @@ public:
 
     bool duplicateSupported() const { return false; }
     Core::IEditor *duplicate(QWidget * /* parent */ ) { return 0; }
-    bool isTemporary() const { return false; }
     Core::Id id() const;
 };