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 bug 80837. For 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