jquery.makeCollapsible improvements

jquery.makeCollapsible improvements
* Check for validness of $defaultToggle only needed once (Follow-up r83309 CR)
* The new jQuery sortable plugin caused a minor breakage in other actions bound within table cells. Since, in contrary to the old wikibits script, the entire <th> is clickable and the triangle is a background image. This means anything that is clickable inside the <th> (such as [<a>Collapse</a>] propagates/bubbles up to the <th> and causes the column to be re-sorted. In other words, collapsing a table caused the last column to be sorted, and when expanded again in the opposite order. Fixed by adding event.stopPropagation(); (note, not "return false" as there may be custom events bound to the anchor tag which should be fired as well. We just need to stop propagation not all other event, such as for customToggles).