Add AppStream data

We should ship AppStream data with Picard. AppStream adds standardized descriptions of applications, that can be used by different application stores / installers. E.h. GNOME Software and KDE Discover make use of it. It is also required for Flatpak packages.

See https://www.freedesktop.org/wiki/Distributions/AppStream/ for details.

Tasks:

1. Add an appdata XML file, use https://github.com/flathub/org.musicbrainz.Picard/blob/master/org.musicbrainz.Picard.appdata.xml as a starting point and extend it with a proper description, more URLs and other suitable data (see AppStream specs)

2. Make proper screenshots (2-3, main window, maybe options), host them on MusicBrainz FTP and link them

3. Make sure the AppData XML gets installed to /usr/share/metainfo