Add 'Music' keyword to desktop file

totem has been GNOME's default music player for as long as I can remember, since Music cannot open music files and Rhythmbox was never part of GNOME core.

Now downstream is finally catching up by removing Rhythmbox: https://pagure.io/fedora-workstation/issue/164

One concern is that users will have difficulty discovering that totem is how you play music. Adding a desktop file keyword is not really a spectacular way to address this, but it's better than nothing.

Design team is currently pursuing a strategy of trying to ensure users understand that music should be played in Videos without redesiging or rebranding the app. I'm not really sure if this is a great approach, but we don't currently have consensus to do more than add the keyword.