Bug 124620 Referenced Parameter icon should be invalid if the parameter in library is deleted
Description: Referenced parameter icon should be invalid if the source parameter in library is deleted. Steps to reproduce: 1.Create a library. Insert a report scalar parameter. 2.Create a report, use the above library. In the library explorer, pull the parameter to the data explorer view of report. 3.Open the library file, delete the parameter. Then reopen the report design file, the icon of parameter seems nothing happened. But if those operations happened in dataset, the icon of dataset in the report will be invalid. It is acceptable. Expected Result: The icon of referenced parameter should be invalid.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/DesignElement.java
* element does not extend from another user-defined element, or 2) the
	 * extends element is undefined.
	 * <p>
	 * Part of: Inheritance system.
	 * 
	 * @return parent element
	 */
	public DesignElement getExtendsElement( )
	{
		if ( extendsRef == null )
			return null;
		return extendsRef.getElement( );
	}
	/**
	 * Gets the name of the element that this element extends.
	 * 
	 * @return The name of the extended element.
	 */
	public String getExtendsName( )
	{
		if ( extendsRef == null )
			return null;
		return StringUtil.buildQualifiedReference( extendsRef
				.getLibraryNamespace( ), extendsRef.getName( ) );
	}
	/**
	 * Returns the optional element name.
	 * <p>
	 * Part of: The name system.
	 *