DMN11 — XSD: add optional @name to inputVariable

In a decision logic expression, inputVariable is an ID reference to an InformationItem, which provides the variable's name. The expression (e.g. binding expression or literal expression) then references the variable by name. This works operationally, but the logic of the XML would be much easier to follow and debug if the inputVariable allowed an optional name attribute to hold the variable's name.

Merged with and Resolved by DMN11-69

I submitted this issue but now I think better approach is to remove id references like inputVariable in tExpression and use names instead. This issue is thus merged with DMN11-69 and resolved by that proposal.