Add AppStream metadata file
Summary:
Alternative to D23302. According to the AppStream documentation (https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html),
adding an AppStream metadata file with multiple launchable tags will prevent AppStream
info generators from parsing desktop files to determine metadata, which sbhould solve
the problem of System Settings showing up twice in Discover and other software centers
due to how it installs more than one .desktop file.
Test Plan: Cannot test AppStream changes, but appstreamcli validate passes, and upon being compiled and installed, the file is located in the right place.
Reviewers: apol, mak, Plasma
Reviewed By: apol
Subscribers: kossebau, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23306