fix comment typo.
DataSelection: Double-click on last row means "add a file"
operator debug names
OpPixelClassification: Moved FreezePredictions slot to the cacheful pipeline, since that's where it belongs (non-cache pipeline doesn't need it)
pixel classification: data export op doesn't bother propagating dirty notifications for label names and colors
OpDataSelection: Removed error for changing dataset dimensionality.
Along with the 'dimensionality-fix' of lazyflow, Fixes #941, for now, anyway...
(Dimensionality inconsistency errors are common and hard to track down,
which is why previously, we opted to simply forbid changes in dimensionality)