YITH WooCommerce Wishlist Premium 3.31.0

* Tweak Prevent undefined index: user_id when user is loggin
* Fix: Fixed active status for default wishlist when WPML is active
* Fix: Fixed active status for default wishlist when WPML is active
* Fix: fixed the default wishlist name in the multi wishlist select
* Fix: widget not recognizing current wishlist when WPML is active
* Fix: notice when sending Promotional email, due to access to legacy attributes
* Dev: new filter yith_wcwl_wishlist_disabled_for_unauthenticated_user_message_condition
* Dev: New action 'yith_wcwl_default_user_wishlist'

* New: WooCommerce 3.6.x support
* New: added a WhatsApp share button on mobile
* New: add new shortcode yith_wcwl_show_public_wishlist
* Tweak: using add_inline_style to include custom css code
* Tweak: no longer adding empty style tag to the page
* Update: Spanish language
* Fix: get the correct value for wishlist name
* Fix: deprecated notice caused by product id attribute being accessed directly

* New: added support to WooCommerce 3.5.4
* Update: internal plugin framework
* Update: Dutch translation
* Fix: added explicit array casting on shortcode to avoid warning
* Fix: don't add custom classes to body if wishlist page is not set
* Fix: changed a wrong method in the enqueue style
* Dev: add filter yith_wcwl_email_share_subject