This script sets the tabindex for a collection of elements.

It can be useful if there are a lot of elements which are generated in some way
and setting the tabindex attribute where is a chore.

For tables it can be useful to set the tabindex based on the cells -> rows from
top-left vertically.  Normally this would be horizontally (left to right).

Note that there are also bugs and inconsistencies in how Firefox handles tabindex.
