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/api/DesignElementHandle.java
ElementPropertyDefn propDefn = getElement( ).getPropertyDefn( propName );
		if ( propDefn == null )
			return null;
		if ( propDefn.getTypeCode( ) != PropertyType.COLOR_TYPE )
			return null;
		return new ColorHandle( this, propDefn );
	}
	/**
	 * Gets the font handle for the element. If this element defines a font
	 * family property, return a <code>FontHandle</code>. Otherwise, return
	 * <code>null</code>.
	 * 
	 * @return a corresponding FontHandle or <code>null</code>.
	 * 
	 * @see FontHandle
	 */
	protected FontHandle getFontProperty( )
	{
		ElementPropertyDefn propDefn = getElement( ).getPropertyDefn(
				Style.FONT_FAMILY_PROP );
		if ( propDefn == null )
			return null;
		return new FontHandle( this );
	}
	/**
	 * Returns the value of an element reference property. Returns a handle to
	 * the referenced element, or <code>null</code> if the reference is
	 * unresolved or unset.
	 * 
	 * @param propName