Enhancement: some scroll improvements regarding #323

This is not a total fix of #323 but offers a few improvements.

Now we display 6 rows of the table instead of 5 and part of another one. When the user clicks on a down arrow, the table scrolls down by one row. When the user turns the mouse scroll wheel one time, the table scrolls down by three rows.

I switched the scroll mode to `JViewport.BACKINGSTORE_SCROLL_MODE` which seems to be giving a little better performance.

I also removed several redundant calls of `CanvasSE.imageChanged()`. This method is already called by `CanvasSE.setSymbol(Symbolizer)`. Every time it was called twice, and the corresponding preview was painted twice.

Also, the scroll bar seems to be keeping up (more or less) with the table. (@gpetit Do you confirm?) It still lags a little, but I think this is due to the previews being redrawn (regenerated?) every time we scroll. It would be best to find a way to not have to regenerate the preview so many times.

Again, not perfect, but a few improvements.