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
*             value is invalid.
	 * @see SimpleValueHandle#setStringValue(String)
	 */
	public void setStringValue( String value ) throws SemanticException
	{
		setValue( value );
	}
	/**
	 * Return the property definition.
	 * 
	 * @return the property definition.
	 */
	public IElementPropertyDefn getPropertyDefn( )
	{
		return this.propDefn;
	}
	/**
	 * Clears the value of the property on every element.
	 * 
	 * @throws SemanticException
	 *             If the value cannot be cleared.
	 */
	public void clearValue( ) throws SemanticException
	{
		setValue( null );
	}
	/**
	 * Returns the element reference value list if the property is element
	 * referenceable type. The list of available elements are sorted by their
	 * names lexicographically.