YITH WooCommerce PDF Invoice 4.25.0

* Tweak: option panel changes
* Tweak: enqueue frontend script only in Checkout page
* Update: updated plugin framework
* Fix: notice array to string conversion in admin order page
* Fix: prevent issue on validation of XML file (Electronic Invoice module for Italian customers)
* Fix: prevent get wrong shipping and fee taxes in the invoice details
* Fix: fixed the price for unit on invoice details template (also affected the discount percentage value)
* Dev: added action hook yith_ywpi_after_generate_template_pdf

* 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