Collapsible set: Do not rely exclusively on the presence of the data-role="collapsibleset" and data-role="collapsible" to identify collapsibles and their parent accordions. Use :mobile-collapsible and :mobile-collapsibleset as well.
Collapsible: Do not collapse already-collapsed collapsibles. Fixes #5911. Thanks @DzenisevichK.

http://jsperf.com/5911-collapsibleset/2
Demos: Updated responsive panels CSS example. Fixes #5936.