Add ability to export top-level groups (folders)

This pull-request adds the ability to export all your top-level groups (folders, or layerSets)

The default option is still to export individual layers, but you can toggle the behavior in the dialog.

There's a decent amount of copy-paste from exportLayers.  I didn't attempt to refactor the exportLayers to support this feature.  I just copied the code in exportLayers and modified it to work.

And I don't really understand how the background images fit into this, so I just commented that part out.