View source for XUL:Trees Cell Selection

One of our clients has requested the following, and we want to develop it on the
trunk and get it checked in. We feel it would be a useful addition to the tree
widget.

1) The ability to have a cell based selector in a tree, allowing us to navigate
by keyboard across a single row to each adjacent column.

2) The ability to attach an event to the tree (or a cell) that would allow us to
know which cell is currently selected when a key (such as enter) is pressed,
based on its row and column.

[Bug]https://bugzilla.mozilla.org/show_bug.cgi?id=296040

Compare [https://bugzilla.mozilla.org/show_bug.cgi?id=80837 bug 80837]. For [https://bugzilla.mozilla.org/attachment.cgi?id=115637 attachment 115637] I added style rules to make it possible for a cell to be selected rather than a row. Unfortunately those rules were not used in favour of a pure CSS solution. You may find you want to reinstate them. --Neil