Fix for Django ticket #17218 (Select Filter ... in a collapsed fieldset)

Tested in Safari/Mac, FF/Mac, IE8, IE7 (via compatibility mode).

If a fieldset is collapsed on initial page display, defer resizing the "to" side of a select filter. Assign a one-time event listener instead, and wait for a `show.fieldset` event to trigger the resize later.

I've adde `show.fieldset` and `hide.fieldset` events (custom jQuery events) to the fieldset toggle in `collapse.js`, which should be useful in similar situations.