Bug 148747 Design become invalid when I extend a masterpage from Library
The attachment contains a report file and a library file. Reproduce: 1. Deploy the library("lib"), and open the report. 2. Drag Masterpage("page1") from "lib" to report. 3. Reopen the report, it become invalid.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/ExtendedItemHandle.java
* 
	 * @return the extension name as a string
	 */
	public String getExtensionName( )
	{
		return getStringProperty( ExtendedItem.EXTENSION_NAME_PROP );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.api.DesignElementHandle#getDefn()
	 */
	public IElementDefn getDefn( )
	{
		ElementDefn extDefn = ( (ExtendedItem) getElement( ) ).getExtDefn( );
		if ( extDefn != null )
			return extDefn;
		return super.getDefn( );
	}
	/**
	 * Loads the instance of extended element. When the application invokes UI
	 * for the extended element, such as listing property values in property
	 * sheet, set the value of the extension-defined properties and so other
	 * operations, the application must create an instance of the extension
	 * element first. The created extended element reads its information cached
	 * by the handle and de-serialize the extension model.
	 * 
	 * @throws ExtendedElementException
	 *             if the serialized model is invalid
	 */