Recommend the VC limitation to documents annotated by a certain foundry/layer, when requested in the query

[Akron]

Whenever a query contains specific annotations possibly not available for all documents (e.g. [mate/l=Baum] in Poliqarp), the user should get a notification that it may be useful to limit the virtual corpus to only contain documents annotated with the requested foundry/layer.
A simple button for this limitation should be attached to the virtual corpus helper.

This requirement may potentially be set by default by the user, meaning that whenever a user does a query, the VC is automatically rewritten in that way.

This feature would need a modifications in Koral and KoralQuery (see KorAP/Koral#27 ).

The feature was requested by franck.