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
* @param propName
	 *            the property name
	 * @return a array containing choices for the given property. Return
	 *         <code>null</code>, if this property has no choice.
	 */
	public IChoice[] getChoices( String propName )
	{
		PropertyHandle propertyHandle = getPropertyHandle( propName );
		if ( propertyHandle == null )
			return null;
		return propertyHandle.getChoices( );
	}
	/**
	 * Returns the short display label for this element.
	 * 
	 * @return the display label of this element in SHORT_LABEL level.
	 * @see #getDisplayLabel( int )
	 */
	public String getDisplayLabel( )
	{
		return getDisplayLabel( DesignElement.SHORT_LABEL );
	}
	/**
	 * Returns the display label for this element. The display label is the
	 * localized display name to be shown in the UI. The display label is one of
	 * the following:
	 * <p>
	 * <ul>
	 * <li>The localized display name of this element, if the display name
	 * resource key is set and the localized string is available</li>