YITH WooCommerce Bulk Product Editing Premium 3.15.0

* New: support for WooCommerce 3.9
* Update: language files
* Update: plugin framework
* Fix: issue when loading downloadable files in plugin table
* Tweak: improved style

* Update: plugin framework

* New - sort products by SKU
* New - filter by 'Allow Backorders'
* New - support to WooCommerce 3.7
* Update - plugin framework
* Update - language files
* Fix - label for 'Stock quantity' field in Filters
* Fix - bulk edit tabs freeze on hover
* Tweak - prevent issue when product is not set
* Tweak - fixed issue when selecting products on filters
* Dev - added yith_wcbep_allow_editing_custom_fields_in_variations filter
* Dev - added yith_wcbep_save_custom_field_{} filter