avoid duplicate notifications from showing up on Windows and remove whitespaces

Summary: without this patch, most notifications show every notification twice. This seems to be an internal problem of knotification, but until that is fixed, this is the next best solution IMHO.

Test Plan:
without this patch, every mobile notification from KDE Connect should show up twice.

Reviewers: Frameworks, broulik

Reviewed By: broulik

Subscribers: nicolasfella, ngraham, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23133