improvements to "Label Point Ids"

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

"Label Point Ids" should only label the visible points, not those occluded by another part of the data set.

Also displaying underlying point data seems more useful than just displaying point ids. Maybe we could allow the user to pick points / cells and display the underlying data for them.

This seems related to but #501.

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