Added definition of DataConstraintError, and handle it appropriately in the DataSelection applet.
Dataset properties editor: Improved UI for specifying data range.
Dataselection: Replaced "Clear Selected" button with a "reset" context menu item.
DataSelection: Allow user to append files from the summary tab.
LayerViewer example workflow:
- Added to the startup screen (for now).
- Added example constraints checking function to OpLayerViewer
Cleanups.