Add support for dynamically updated facets

[sekjal]

Some facet interfaces are enriched by updating the facets after each new Ajax request is performed. This commit adds support for this.

The key is to use Event Delegation (learn.jquery.com/events/event-delegation) to bind the listener to an unchanging parent element (the form itself) rather than directly to the inputs. This way, inputs can change with the Ajax requests, and the binding is not broken.

Implementation requires a custom postAJAX function to be specified in the plugin configuration, and for the updated facets to be returned in some format that can be processed by that function.