annoying playlist list scroll reset fix

so, I have quite a few albums and playlists saved now, and it was annoying me that every time I wanted to add a song to a playlist, it would actually reset the sidebar's scroll.

in this fix, I simply saved the scroll and restored, which I think should be done every time you render partials... but an additional improvement can be made too, just simply checking to see if the data has changed, and if nothing is changed, no rerendering needs to be done.

ok man. peace