YITH Stripe Connect for WooCommerce Premium 2.31.0

New: support for WooCommerce 4.9
Update: plugin framework
Fix: possible fatal error happening in rare cases when processing payment

* Update: plugin framework
* Tweak: added back \YITH_Stripe_Connect_Frontend::stripe_connect_account_page method as deprecated, to improve compatibility

* New: support for WordPress 5.4
* New: support for WooCommerce 4.0
* New: support for API 2020-03-02
* Update: Stripe PHP library to version 7.27.2
* Update: plugin framework
* Update: Dutch language
* Tweak: improved check over WooCommerce existance
* Tweak: idempotency key now is generated basing on order id
* Tweak: code refactor
* Dev: added new function to get charge object
* Dev: added new function to get balance transaction object