Cleanups: OutputSlots shouldn't be optional.
OpDataExport: Added progressSignal member
LayerViewerGui: Clean up layers that aren't needed, and cleanUp SingleChannelSelector helper operators with their associated layers.
Also, minor refactoring of determineDatashape to make it easier to override in subclasses.
Added DataExport applet and gui (Partly based on a copy of the old batch applet gui)