Add example illustrating implementation of aria-actions

[mcking65]

Add an experimental variation of one of the examples of tree, tabs, or listbox that implements the aria-actions property proposed in w3c/aria#1805.

The example should be added to APG PR #2977, which also implements the APG infra to support experimental content defined in issue #2836.

Note: in #2977, the file 'content/patterns/treeview/examples/treeview-1c.html' is just a placeholder. This file could be renamed and used as the basis for the new example, or this file could be removed in favor of using a different example as the foundation for the new example of aria-actions.

[mcking65]

Replaced with #3070.