YITH WooCommerce PDF Invoice 4.16.0

* New: support for WordPress 5.3
* New: support for WooCommerce 3.8
* New: added new option to show or hide broken down taxes in the invoice summary
* Update: plugin framework
* Update: updated Dutch language
* Update: updated Italian language
* Fix: fixed product price and sale price on invoice details
* Fix: prevent to get wrong tax when there is two or more taxes
* Dev: new filter 'yith_ywpi_get_item_price_per_unit_sale' and 'yith_ywpi_get_item_price_per_unit'

* Update: updated .pot file
* Update: updated Dutch language
* Fix: fixed the automatic document generation on the order creation
* Fix: prevent warning on invoice fields in admin view
* Dev: fixed minor issue with the tax label
* Dev: added some changes in the placeholders calls

* New: support WooCommerce 3.7
* Update: updated Italian language
* Update: updated plugin core
* Fix: fixed the automatic packing slip generation
* Fix: fixed the bulk regenerate documents feature
* Fix: avoid additional tax rows in the total if the tax is zero
* Dev: new filter to change the VAT field name in the checkout
* Dev: fixed a warning with WC 3.7
* Dev: added a new filter ywpi_allow_attach_credit_note
* Dev: changed the method yith_get_prop to get_post_meta in the replace_customer_details_pattern method