Move displayName from IEditor to IDocument

The display name is not editor instance specific, but belongs to the
document.

Change-Id: I3c936f04a86e10e6ca30063d85036d85b4b5880e
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/diffeditor/diffeditor.h

@@ -63,8 +63,6 @@ public:
     bool createNew(const QString &contents);
     bool open(QString *errorString, const QString &fileName, const QString &realFileName);
     Core::IDocument *document();
-    QString displayName() const;
-    void setDisplayName(const QString &title);
     Core::Id id() const;
     bool isTemporary() const { return true; }
     DiffEditorWidget *editorWidget() const { return m_editorWidget; }
@@ -86,7 +84,6 @@ private:
     Internal::DiffEditorFile *m_file;
     DiffEditorWidget *m_editorWidget;
     QComboBox *m_entriesComboBox;
-    mutable QString m_displayName;
 };
 
 } // namespace DiffEditor