Collapsible: Elaborating on the semantics behind $.mobile.collapsible.defaults.

Also, perf stats for the review so far: http://jsperf.com/collapsible-widget-review
Collapsible: Implement public methods expand() and collapse() for programmatic manipulation and trigger namespaced signals via this._trigger() instead of non-namespaced ones.

This obviates the need for handling programmatic expansion/collapse via a non-namespaced signal triggered on this.element.
Collapsible set: Start using the new expand()/collapse() public API instead of the signal-based one. Fixes #5271, #5798.