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/imageviewer/imageviewer.cpp

@@ -148,11 +148,6 @@ Core::Id ImageViewer::id() const
     return Core::Id(Constants::IMAGEVIEWER_ID);
 }
 
-bool ImageViewer::isTemporary() const
-{
-    return false;
-}
-
 QWidget *ImageViewer::toolBar()
 {
     return d->toolbar;