YITH WooCommerce Ajax Product Filter Premium 5.5.0

* New: Support for WordPress 5.4
* New: Support for WooCommerce 4.0
* Update: plugin framework
* Fix: Undefined index prices

* New: Support for WooCommerce 3.9
* Fix: Missing current tag in Stock and On sale filter
* Fix: Not updating filters when the customer apply a filter by price

* Tweak: Prevent plugin query in non WooCommerce pages on frontend
* Update: plugin framework
* Fix: Removed error log
* Fix: Variable $li_printend undefined on frontend