[CHANGE] HTML ToC expander icons

[Rob Beezer]

Peter Siebel has suggested using +/- ("add"/"remove") icons in the HTML Table of
Contents when you expand a level (and pack it away again). Some still prefer
the rotating chevrons.

Now we have both. +/- is in use now with the Salem theme, so you will see that
change after updates and a rebuild. This is a theme-stylist option, not a
publisher option. So register a preference with your favorite theme stylist.

True joint effort with Peter S, Andrew S, and Rob B.

Rob