add snoretoast backend for KNotifications on Windows

Summary: This is the main patch that adds the interfacing between KNotifications and SnoreToast.

Reviewers: broulik, sredman, vonreth, albertvaka

Reviewed By: sredman, vonreth

Subscribers: nicolasfella, pino, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21661