Cleaner selection.

Storing the jQuery object for the selected cell in a global variable,
along with that cell’s row and column indices.

Also started experimenting with styles for showing the
currently-selected cell.  That’s what I’d meant to accomplish; I ended
up doing the above as a side effect, and it seems solid enough to check
in as a standalone piece.

In general, I check stuff in when I have something solid, which I feel
other people can check out and run without breaking the project.