Increase performance of mass rating changes.

Currently in Clementine if you wish to select many songs in a playlist and then set them all to the same rating, it is very slow. It calls SQL update for each and every song selected instead of calling update once. So I added a new set of methods to take a list of song IDs and one rating. It will update all those songs to that rating in one SQL update call.

How can I test the slowness of Clementine?
1.) Have a playlist with 20 or more songs
2.) Select all songs
3.) Change the rating on one and it will change all that are selected
4.) Watch it slowly change the ratings...you'll also notice your hard drive light on the whole time

This should be almost instantaneous but it's not currently.

Please let me know if my style is bad...I'm used to doing Java not C++