Bug 123406 Add geLocaltStringProperty in GroupElementHandle and GroupPropertyHandle
Please refer to 123405. GUI need a way to get the local setting of given property because current getStringPropery method will return inherited value if there are no local value.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/GroupPropertyHandle.java
* equal within the collection of elements. The value return are localized.
	 * 
	 * @return The localized value as string if all the element values for the
	 *         property are equal. Return null, if elements have different value
	 *         for the property.
	 * @see SimpleValueHandle#getDisplayValue()
	 */
	public String getDisplayValue( )
	{
		if ( !shareSameValue( ) )
			return null;
		
		
		List elements = handle.getElements( );
		return ( (DesignElementHandle) elements.get( 0 ) )
				.getDisplayProperty( propDefn.getName( ) );
	}
	/**
	 * Set the object value on a group of elements. This operation will be
	 * executed within a transaction, it will be rollbacked if any set operation
	 * failed.
	 * 
	 * @param value
	 *            the object value to set
	 * @throws SemanticException
	 *             if the property is undefined on an element or the value is
	 *             invalid.
	 * @see PropertyHandle#setValue(Object)
	 */
	public void setValue( Object value ) throws SemanticException