OpDataSelection: Ensure that the special 'Image' slot is NOTREADY if there are no images loaded.
OpDataSelection: Resize DatasetGroup immediately when DatasetRoles is set.
Batch: gui: Set 'Dirty' flag before setting up preview, to avoid making the output layer ready too soon.
Fixes #370
Batch gui: Use @threadRouted to ensure that the table is always updated in the gui thread.
Fixes #386
Pixel Classification: Added a constraint to ensure that the input images all have the same dimensionality.
DataSelection: Imported stack volume datasets use human-readable filePaths.