tests: Added @timeLogged to pixel classification labeling tests.
DataSelection: Respond to double-click of an empty table.
Serializer: SerialBlockSlot: Before serializing nonzero blocks, optionally shrink each block to its nonzero bounding-box.
LayerViewerGui: When a slot's metadata changes, don't bother updating the layerstack until the gui is visible.
(Saves a lot of wasted updates.)