Accelerate the updater progress

In particular when checksumming very many files, it does not make too
much sense to update the progress dialog slavishly: the GUI update takes
substantially more time than the action whose progress is to be reported
in that case. So let's just skip it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add the "Update Site" column to the update table

This addresses https://github.com/imagej/imagej-ui-swing/issues/4.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>