Bug 188578 Add "displayColumnName" on TabularLevel
Add Property "displayColumnName" on TabbularLevel and corresponding APIs on handle. Property Type: expression.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/writer/ModuleWriter.java
* 
	 * @see org.eclipse.birt.report.model.elements.ElementVisitor#visitTabularDimension(org.eclipse.birt.report.model.elements.olap.TabularDimension)
	 */
	public void visitTabularDimension( TabularDimension obj )
	{
		writer.startElement( DesignSchemaConstants.TABULAR_DIMENSION_TAG );
		super.visitTabularDimension( obj );
		writer.endElement( );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.elements.ElementVisitor#visitTabularHierarchy(org.eclipse.birt.report.model.elements.olap.TabularHierarchy)
	 */
	public void visitTabularHierarchy( TabularHierarchy obj )
	{
		writer.startElement( DesignSchemaConstants.TABULAR_HIERARCHY_TAG );
		super.visitTabularHierarchy( obj );
		property( obj, ITabularHierarchyModel.DATA_SET_PROP );
		writeSimplePropertyList( obj, ITabularHierarchyModel.PRIMARY_KEYS_PROP );
		writer.endElement( );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.elements.ElementVisitor#visitTabularLevel(org.eclipse.birt.report.model.elements.olap.TabularLevel)
	 */
	public void visitTabularLevel( TabularLevel obj )
	{
		writer.startElement( DesignSchemaConstants.TABULAR_LEVEL_TAG );