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
checkAllowedOperation( );
		propName = StringUtil.trimString( propName );
		
		ElementPropertyDefn prop = element.getPropertyDefn( propName );
		if ( prop == null )
			throw new PropertyNameException( element, propName );
		setProperty( prop, value );
	}
	/**
	 * Sets the value of a property. If the mask of a property is "lock", throws
	 * one exception.
	 * <p>
	 * If the mask of this property has been set to "lock", no value will be
	 * set. To set the value of a property, the mask value must be "hide" or
	 * "change".
	 * 
	 * @param prop
	 *            definition of the property to set
	 * @param value
	 *            the new property value.
	 * @throws SemanticException
	 *             if the value is invalid or the property mask is "lock".
	 */
	public void setProperty( ElementPropertyDefn prop, Object value )
			throws SemanticException
	{
		checkAllowedOperation( );
		
		if ( prop.isElementType( ) )
		{