lp945091 save columns independently for hold lists

lp945091 save columns independently for hold lists

Teaches util.list.init a new parameter, columns_saved_under, which effectively
replaces the tree/list id attribute when used to construct a filename for saving
and retrieving column settings.

Use this technique with holds.js so that each XUL-based hold list interface can
save column configurations independently.

Specifically, these interfaces:

Actions for this Record -> View Holds
Patron Display -> Holds
Circulation -> Browse Hold Shelf
Circulation -> Pull List for Hold Requests

Signed-off-by: Jason Etheridge <jason@esilibrary.com>