[repository schema] Support mixed expression/script languages

[donmendelson]

Currently, the selection of expression language is global to an Orchestra repository at its root element, and its default is the Score DSL. It affects the interpretation of conditional expressions in <when> elements and values in <assign>.

<xs:attribute name="expressionLanguage" type="xs:string" default="Score">
	<xs:annotation>
		<xs:documentation>The syntax of 'expressionType'</xs:documentation>
	</xs:annotation>
</xs:attribute>

Some projects may need a mix of expression or script languages for different purposes. Therefore, I propose to add the expressionLanguage as an optional attribute of of <when> to override the global setting.