Simplify grid DOM attribute prefix, remove dead code.
Fix grid editing.

Broken by the introduction of cell contents wrapper elements, because the
event target wasn't associated with a grid cell any more.

Introduce proper Cell objects for grid cells, linking to the TD element
and the wrapper DIV element.
Handle com.qwirx.data.DiscardBlocked exceptions.

Instead of forcing Cursor to have a way to suppress them, which results in
inconsistent behaviour and the need to check the return value from
maybeDiscard(), defeating the point of throwing an exception in the first
place.