YITH WooCommerce PDF Invoice 4.16.0

* New: Support for WooCommerce 4.0
* New: Support for WordPress 5.4
* New: Fields for electronic invoice xml document
* New: Options for electronic invoice
* New: Options for electronic invoice (third intermediary)
* New: A "Private/Company/Freelance" field in checkout when the electronic invoice is enabled
* New: Support for Khmer Unicode
* New: JS validation on SSN for italian customers
* Update: Plugin framework
* Update: Language .pot file
* Update: Italian translation
* Update: Updated MPDF to 8.0.5
* Fix: PEC destinatario field always shown
* Fix: Placeholder not replaced in credit notes
* Fix: Changed the method to add the footer with mpdf library
* Fix: Fixed a wrong percentage tax value in the invoice
* Dev: Added a self redirect to the regenerate button
* Dev: All string scaped

* New: support for WooCommerce 3.9
* Tweak: Check if rtl language before print the document
* Update: .pot file
* Update: Dutch language
* Update: Spanish language
* Dev: new filter 'ywpi_next_progressive_file_id'

* 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