Editors: Provide a base implementation for duplicate{,Supported}

Helps avoiding trivial re-implementations.

Change-Id: Iaa2e6b92503a2b9553432568c2047c28899c7ea6
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/imageviewer/imageviewer.h

@@ -61,9 +61,6 @@ public:
     QString displayName() const;
     void setDisplayName(const QString &title);
 
-    bool duplicateSupported() const;
-    IEditor *duplicate(QWidget *parent);
-
     QByteArray saveState() const;
     bool restoreState(const QByteArray &state);