Listbox clutters analysis display with empty choices

The current analysis function lists all choices with tallies for all select components. This may be a very large number of choices particularly for ListBoxes which are generally selected *because* there are two many choices to render as checkboxes or radio buttons.

Given that the selection in a Listbox are generally much fewer than the non-selections this patch addresses this problem by only displaying Listbox choices within the analysis function where they have a raw tally greater than zero. ie, one or more people have selected them.

It works by flagging non-zero tallies in the shadow row table and then unset()ing them prior to renderring. Choice numbering remains unchanged but because zero tally elements are removed, may be unsequential.

The covariance matrix will have a similar problem in two dimensions but this is not addressed by this patch and is the subject of a separate feature request. This is deemed less critical because the covariance matrix can be turned off.

This issue is related in nature to the csv column issue but is code unrelated.