Collapsible: Renaming variable "o" to "opts" for options.
Collapsible: Remove unneeded variables.
Collapsible: Shortening unnecessarily long variable names.
Collapsible: When replacing a legend, enhance the replacement before inserting it into the DOM.
Collapsible: Make _setOptions work with possible inheritance from an accordion.
Collapsible: Add refresh() method which basically applies an empty set of options.
Collapsible set: Declare the same options as collapsible, and update child collapsibles when any of those options change.
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.
Collapsible: There is no need to enhance-depend on page and toolbar.

This reverts the change that was introduced in 2af091d5adbc7d22c567da1da6e1463ee044e107.
Collapsible: Collecting UI-related variables into this._ui.
Widget base class: Moving _toggleClasses from collapsible.
Collapsible set: Reflect options that affect the collapsibleSet instead of merely forwarding them all to the children.
Collapsible: Implement _destroy().
Collapsible: Detach heading before performing a slew of DOM manips on it and re-attach it afterwards.
Collapsible set: Implement _destroy().
Moving function domEqual to testHelper.
Tests: Collapsible: Convert to sync tests.
addFirstLastClasses: Write the corresponding remove-method.