YITH WooCommerce Product Add-ons Premium 4.10.1

* Fix: Activation function
* Fix: Free version compatibility
* Fix: Minor bugs

* New: German translation
* Tweak: Improved Multi Vendor compatibility
* Update: YIT Plugin Framework 3.0.21
* Fix: Avoid undefined "first_options_free_container" variable
* Fix: Add-ons position in variable products
* Fix: Required single textarea
* Fix: Strings translation bug
* Fix: Vendor in group settings
* Fix: Data js error
* Fix: Minor bugs

* New: "First X options free" feature
* New: "Hide Label" setting for add-on options
* New: Support to "Variable subscription" products
* Update: YIT Plugin Framework 3.0.20
* Tweak: Improved WPML compatibility
* Fix: Add-ons position in variable products
* Fix: Vendor settings in groups
* Fix: Show Options shortcode
* Fix: Main image replacement
* Fix: Filter to disable the plugin init
* Fix: Groups categories with WPML
* Fix: Minor bugs