Bug 197589 Need constant for automaic attribute name "DateTime"
Currently when set dateTimeLevelType for cube level, model will automatically create a built-in attribute "DateTime". But the name is hard-coded. We need a public constant for this name to allow UI to access it.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/command/PropertyCommand.java
{
				IReportItem extElement = extendedItem.getExtendedElement( );
				if ( extElement == null )
					return;
				extElement.checkProperty( prop.getName( ), value );
				extElement.setProperty( prop.getName( ), value );
				return;
			}
		}
		if ( element instanceof Level
				&& prop.getName( ).equals( ILevelModel.DATE_TIME_LEVEL_TYPE )
				&& value != null )
		{
			ActivityStack stack = getActivityStack( );
			PropertyRecord record = new PropertyRecord( element, prop, value );
			record.setEventTarget( getEventTarget( prop ) );
			stack.startTrans( record.getLabel( ) );
			stack.execute( record );
			ComplexPropertyCommand cmd = new ComplexPropertyCommand( module,
					element );
			ElementPropertyDefn attributesPropertyDefn = element
					.getPropertyDefn( ILevelModel.ATTRIBUTES_PROP );
			Structure struct = null;
			if ( element instanceof TabularLevel )
			{
				LevelAttribute attibute = new LevelAttribute( );
				attibute.setName( "DateTime" ); 
				attibute
						.setDataType( DesignChoiceConstants.COLUMN_DATA_TYPE_DATETIME );
				struct = attibute;