tagger needs improvements

The tagger in trunk works well but still needs a few improvements to be fully usable:

 * The tag list needs to use a scrollbar on large tag sets instead of expanding the window, since many displays will nto have enough space to fit the expanded window.
 * When saving, a progressbar should be displayed as saving data to many files can be very slow. A simple "x/xx written" like ex falso uses would suffice.
 * There's no way to remove a tag from multiple tracks, since the remove button causes the "apply to multiple" button to disappear too. Perhaps just show this with a strike-through instead of completely removing?