Fixed "Show but delay execution" filters on search results showing an empty filter after loading. The REST endpoint was returning the built filter correctly, but the JavaScript that swaps it into the page was looking for a CSS class the built response only carries in some widget configurations, so for plain configurations the placeholder was removed without the options being inserted. The swap now targets the content element the endpoint always returns, and the freshly loaded filter is re-initialised (selected counts, per-page term lists and crawler-safe links) after insertion. Thanks to @rexhavoc for tracing this and sending in the diagnosis
* Fixed a delayed filter getting permanently stuck in its loading state if the load request failed - it can now be retried by interacting with the filter again
* Fixed a fatal error when clearing the filter cache on sites where the transient widget cache constant is not defined
