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/imageviewer/imageviewer.h

@@ -58,8 +58,6 @@ public:
     bool open(QString *errorString, const QString &fileName, const QString &realFileName);
     Core::IDocument *document();
     Core::Id id() const;
-    QString displayName() const;
-    void setDisplayName(const QString &title);
     bool isTemporary() const;
     QWidget *toolBar();