Update indexing progress less often

wxTheApp->QueueEvent is sufficiently slow that updating the progress
could sometimes take longer than the actual indexing.
Make progress bars less jerky
Improve audio loading progress reporting

Even without the animation the reported progress was an average of 1.5
blocks behind the actual progress, and with the animation it was two
blocks behind. Assuming constant-speed audio decoding it should now be
nearly accurate (it still ignores that the last block may be smaller
than the rest).