xul list secondary sorting

xul list secondary sorting

For xul-based lists in the staff client (interfaces with a grey background),
normal behavior is to sort the list when you left click a column header.

This is still the case, but now if you control+click a column header, it sets
up that column as a secondary sub-sort.  Control+clicking more columns will
set up subsequent sub-sorts.

Normal click a column header for primary sorting will clear out all previously
defined sub-sorts.  Control+clicking when a primary column has not yet been
normal clicked effectively does nothing.

Signed-off-by: Jason Etheridge <jason@esilibrary.com>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>