YITH WooCommerce Wishlist Premium 3.31.0

* New: support for WordPress 5.3
* New: support for WooCommerce 3.8
* Update: plugin framework
* Update: Italian language
* Update: Dutch language
* Tweak: wrapped Add to Wishlist button label into span HTML tag
* Fix: removed occurrences of deprecated properties on promotional email class
* Dev: added new yith_wcwl_product_already_in_wishlist_text_button and yith_wcwl_product_added_to_wishlist_message_button filters
* Dev: added new yith_wcwl_out_of_stock_label and yith_wcwl_in_stock_label filters

* New: WooCommerce 3.7.0 RC2 support
* New: input to copy wishlist link and share it anywhere
* Update: internal plugin framework
* Update: Italian language
* Fix: redirect url if there is more than one parameter on the url
* Fix: changed escape for share link, to properly escape url special characters

* Update: internal plugin framework
* Tweak: improved performance on wishlist page, when user is a guest and store has huge catalog (thanks to Dave)
* Dev: add filter yith_wcwl_wishlist_correctly_created on add_wishlist function