gnome-music.desktop: Rename desktop file

Up until now we were using gnome-music.desktop as name for the desktop
file.

However this is discouraged since it's not using a unique id similar to
dbus addresses.
This also bring the problem that to build with flatpak the desktop file
has to match the dbus address.

So simply rename the desktop file to org.gnome.Music.org

https://bugzilla.gnome.org/show_bug.cgi?id=779905