[lmi] Allow direct drill-down editing in census manager

[Greg Chicares]

This topic:
  https://savannah.nongnu.org/support/?105977
  "Allow direct drill-down editing in census manager"
deserves its own separate thread.

Let me start by asking whether it makes sense to replace the listview
with an appropriate grid-type control as a first step (that we can
deliver separately), then enable drill-down editing as a second step.
We might also defer editing of "input sequences" until a third step
that implements this:
  https://savannah.nongnu.org/support/?104480
  "Pop up alternative input-sequence editor"
Maybe there should even be a zeroth step to clean up class CensusView.