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/diffeditorfile.cpp

@@ -43,6 +43,7 @@ DiffEditorFile::DiffEditorFile(const QString &mimeType, QObject *parent) :
     m_modified(false)
 {
     setDisplayName(QCoreApplication::translate("DiffEditor", Constants::DIFF_EDITOR_DISPLAY_NAME));
+    setTemporary(true);
 }
 
 void DiffEditorFile::setModified(bool modified)