[lmi] CensusView cell-deletion anomaly

[Greg Chicares]

Try this:
  File | New | Census
  Census | Add cell
  Census | Delete cell(s) | Yes
Now "Census" commands that require a single selection should be available,
but are not. Furthermore, attempting
  Census | Delete cell(s)
again should produce a messagebox saying I can't delete all cells, but
instead I see:
  Assertion 'n_sel_items < n_items' failed.
  [file /lmi/src/lmi/census_view.cpp, line 869]
The remaining cell has a selection highlight, but seems not to function as
a selected cell should; left-clicking it "fixes" those problems, though it
has no visible effect on the highlight.