Dont animate items that when neithe rthe old or new y position is within the visible...

Dont animate items that when neithe rthe old or new y position is within the visible area of the playlist. This change actually makes the playlist usable with a large number of tracks ( I tested it with just over 3000 tracks ). The only really bad performance problem with the playlist right now is collapsing and expanding albms wiht a large umber of tracks. 10-20 tracks are almost instantanious, but collapsing someting with about 100 tracks (yeah, kind of a corner case) takes 10-20 seconds...

git-svn-id: svn+ssh://svn.kde.org/home/kde/trunk/extragear/multimedia/amarok@726256 283d02a7-25f6-0310-bc7c-ecb5cbfe19da