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 misc/org.kde.filelight.appdata.xml validate

Reviewers: jriddell

Reviewed By: jriddell

Subscribers: jriddell, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D24385