Enhance "Open Type" to navigate to the .isis file rather than the .java file

[danhaywood]

Otherwise the workflow is rather tricky, requiring to navigate from the generated java class back to the isis source.

[tjuerge]

The "Open Type" action is contributed by JDT. This action is related to Java types only.

So we have to come up with our own "Open Isis Type" action and corresponding view instead.