More control over Relations

**Is your feature request related to a problem? Please describe.**

Currently, there's no way to:
* specify which Label types may participate in which Relations (e.g., a Spouse Relation should only be allowed to link Labels of type Person, not Labels of type Organization)
* specify that a Relation is required for some Labels (e.g., each word is required to have a dependency Relation to its grammatical parent).

**Describe the solution you'd like and alternatives you've considered**

I don't know what's easiest to implement, and I'm not particular about the exact solution. To achieve the constraints on Relations, some possibilities might be:
* Relation could allow a new attribute ``
* As above, but implemented as a new child element, say, ``
* As either of the two above, but implemented in Relation instead of Relations.

To achieve required relations, some possibilities might be:
* Allow `Relations` to be placed within a `View` with `whenLabelValue`, so that the labeling config can determine when the relations panel should show up.
* As above, but keep the global relations panel and just display the subset of the relations panel that applies to the `Label` when you're in the `View` with `whenLabelValue`.