Update org.kde.amarok.appdata.xml

This essentially pastes the description of the repo in the Appstream Summary, since it works well enough for showing up on application stores like Discover. It is fine for merge as is I think.

However, if Amarok is getting revived soon™, I think the summary can be changed on a major release to something more creative and significant, like "The powerful music player ‒ Reawakened" or something like that. Sounds cheesy, but at least users who know about it would casually see it on Discover and notice there was an update, picking their interest in downloading it. What do you think?