work around Scripting Bridge slowness introduced in 10.8.3

This is more or less the same work-around used in 9597c0bfa738cddc386c7a85ee1e6fbab0ae363f with some improvements. Getting the arrays of all tracks and all artists adds about .07 seconds, but using blocks to filter the tracks by artist/album artist reduces the total time by 3-10 seconds.
bump version to 2.3.2