XAS: Submission blocker for element select panel

**Is your feature request related to a problem? Please describe.**
Currently, the element select panel for the XAS plugin does not block submission if no elements are selected for calculation.

**Describe the solution you'd like**
Implement a submission blocker for the `Setting` panel of the XAS plugin. At an initial glance, this should already be possible and will only require a little bit of development/testing to implement.