have 1024x1024 app icon as fallback icon in Snore

Summary: by this patch, apps with a set WindowIcon will have that icon as fallback icon for their notifications invoked through NotifyBySnore backend for KNotifications on Windows 10.

Reviewers: Frameworks, broulik, sredman

Reviewed By: broulik, sredman

Subscribers: sredman, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23196