YITH Quick Order Forms for WooCommerce Premium 1.2.16

* New: Integration with YITH WooCommerce Request a Quote Premium
* Dev: added filter yith_wc_qof_frontend_responsive on $responsive_js variable
* Tweak: avoid PHP Notices
* Update: plugin framework
* Update: language files

* Update: plugin framework
* Update: plugin description

* New: Support to WooCommerce 3.5.0
* Tweak: remove menu tittle for translation
* Tweak: change parent page from yit_plugin_panel to yith_plugin_panel
* Update: Dutch language
* Update: plugin fw
* Fix: prevent php notice
* Dev: filter yith_wc_qof_frontend_responsive on ajax front page