ally.query.tabsequence: tabindex sorting within ShadowRoot

[rodneyrehm]

Because ally.query.focusable can find shadowed elements, util/sort-element-by-tabindex must be extended.

• The tabindex of an element within a ShadowRoot does not affect the global tabbing order, only the one within the ShadowRoot.
• Firefox (currently) ignores positive tabindex [tabindex="1"] within ShadowRoot - these elements are not added to the tabbing order. Firefox sorts tabindex globally, not limited to ShadowRoot.

util/sort-element-by-tabindex should sort the children of a ShadowRoot independently of the document. Within a document, the ShadowRoot must be treated as an immutable group.

[rodneyrehm]

Of course it is only Chrome sorting tabindex within a ShadowDOM, while Firefox sorts them globally, as can be whitnessed in this fiddle (sorry, jsbin is down at the moment…)