Consider rewriting song-table's row-dragging code

[derat]

The row-dragging code in the song-table component has various bugs, e.g. #45 and #49. These seem to come from the awful HTML5 drag-and-drop API, and I'm not sure if it's even possible to fix them.

Maybe I should ditch the API and instead use the approach described at https://www.redblobgames.com/making-of/draggable/. I'd probably need to write some new code to do things that the API already handles, like deciding when to start the drag (ugh, things like the drag threshold seem like they should live in the platform) and scrolling the table (which the API already doesn't do correctly).