Add Windows Store link in AppStream metadata

Summary: This should allow kde.org/application to display a link to the windows when using Windows.

Test Plan: appstreamcli validate org.kde.kstars.appdata.xml

Reviewers: KStars, mutlaqja

Reviewed By: KStars, mutlaqja

Subscribers: mutlaqja, jriddell, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D24386