Better support for invariant testing

[brianpos]

When using the Validate button (which essentially checks the fhirpath expression as though it was an invariant) there should be another fhirpath expression that indicates the context at which to perform the check (unless the location of the selection in the resource is used (as that is now possible - its shown in the status line)

[brianpos]

This is likely to be in the next release, have functionality working, just usability testing the UI for it.