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/genericprojectmanager/genericprojectfileseditor.h

@@ -75,7 +75,6 @@ public:
     Core::Id id() const;
     bool duplicateSupported() const;
     Core::IEditor *duplicate(QWidget *parent);
-    bool isTemporary() const { return false; }
 };
 
 class ProjectFilesEditorWidget : public TextEditor::BaseTextEditorWidget