conflicting id value in elements across mulitple shtml

[SmartLayer]

Pull request #119 revealed a related issue. In the old XSLT model, the id attribute of html elements are only assembled into an HTML file when needed. In the current mode, thus multiple xslt files can have elements of the same type. With the switch away from xslt, shtml, on the other hand, is an internal part of TokenScript and can't have duplicated values in id element. None of our current working examples have hit this issue, but if unsolved, in the future someone will get stuck there - his TokenScript doesn't validate as he used one value for different id attributes in different shtml.

https://docstore.mik.ua/orelly/xml/schema/ch08_02.htm showed a way to modify schema which probably can be used to change the type of id element from xs:ID to a normal identifier without uniqueness constraint. We can attempt that as a fix in 2019/05 namespace.