add system integration to NSIS installer of KDE Connect

Summary: https://i.imgur.com/4OYaaWW.jpg

Test Plan: install this to test : https://drive.google.com/file/d/1WAYnVDrgXvWR6EQ50tOgfFdQKv_fTC7s/view?usp=sharing

Reviewers: vonreth, albertvaka, sredman

Reviewed By: vonreth

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D23577