Just suppress adding impossible paths.

The chances of someone seeing this are slim, it would take getting
a non-raw config and trying to apply parameters after the fact,
which technically someone could do, so I'd like to indicate the the issue
somehow...
Added a note.
If we can't make it work all the way, don't make it work halfway.

Forget trying to translate paths. Just substitute in a marker parameter
that has no paths. I am displeased with this, but right now I think
it's the best option.
ReferenceConstraints annotation.

Similar to editor/FileConstraints.

I was going to put this in the /editor/ package, but examining this annotation
will be part of config validation so I guess it belongs here.
Rejiggered so that additional filters can be added.

A panel is created for them so that the user can at least see that they're
getting a filtered view, but the filters may or may not be user-removable.
Support ReferenceConstraints for picking configs.

Simply annotate your ConfigReference or List<ConfigReference> property
with the subtypes that are acceptable.
Note that this is slightly different from EditorTypes, as that explicitely
lists all classes, whereas this permits any subclass of the specified types.

Examples:
    @ReferenceConstraints({ ActorConfig.Monster.class }) // only allow monsters

    @ReferenceConstraints(description="m.big_guys",
            value={ ActorConfig.HugeMonster.class, ActorConfig.BigBoss.class })