Improve the layout design

The layout is not relative positioned within a main fixed block. This
is better than using several absolute and fixed boxes as it will likely
break less.
Keep width when dragging an artist or album

Previously the album or artist would get wider when dragged, now it
does not. Also add a distance to the song draggables for consistency.
Change the indication when music is being dragged and dropped

This adds a class change to the playlist when something begins being
and another change when that something moves over the playlist.