ListDecorator and PaginatedList shouldn't return the inner list

[GuySartorelli]

Most methods in ListDecorator (and subsequently in PaginatedList and any other subclass of ListDecorator) return the inner list instead of returning itself - e.g. ListDecorator::filter() returns the inner list.

Instead, these should do one of the following:

1. Make a clone of itself, set the inner list of the clone to be the result of the filter/sort/etc, and return the clone
2. Set its own inner list to be the result of the filter/sort/etc and return itself