DMN11 — use refs in XSD only to substitution groups

use <element name=xx, type=tYY/> instead of <element ref=zz/>
This will more consistently name subelements as MM attributed names, not as MM class names. This is a stylistic issue only.

Consistently use MM names in XSD

Change XSD to use attribute names as defined in meta model aggregations.
Remove references to global elements where no polymorphism is needed.

In particular the changes are:

1. remove unneeded global elements and refs
2. Revert back to ref="Import" due to polymorphism with ImportedValues
3. Remove unused element text
4. Use attribute names from meta model
5. Consistently prefix type names with t
(This is only XSD cosmetics and does not affect the actual element names in instance documents)
6. Consistently name elements with MM class name with lower-case first letter
(This last change might be a little controversial, as it affects many element names. However, the previous changes result in a mixture of upper and lower case element name, which certainly will be confusing for tool vendors. Especially, since we see in the OMG BPMN Model Interchange Working Group, that many tool vendors are not performing schema validations, we might end up with subtle differences in import/export that prevent interchange. Also when developing tools to analyse or transform DMN files using XSLT or XPath, predictable element names will safe a lot of hassle.)
7. imported values can only be used in literal expression.