Make histogram update live in threshold plugin

This might be unnecessary. But maybe we want the threshold plugin to update its histogram when its dataset is changed by someone else. It could track dataset updated events and recalc histogram as needed.

Migrated-From: http://trac.imagej.net/ticket/1688

Locate the threshold plugin and the dataset update-event mechanism. Read how the plugin currently builds its histogram, then determine how external dataset changes are reported and whether recalculation can be triggered safely. Done means the histogram updates when another component changes the dataset, without unnecessary recalculation.

Written by the indexing model from the issue text.