Editorial: clean up small parts of sequential focus navigation

[domfarolino]

The sequential focus navigation infrastructure could likely use a major revamp, to make it more algorithmic, and to deal with data structures more directly instead of the many various "conditions" that describe data structures. Before this is accomplished, this PR cleans up some low-hanging fruit by:

• Getting rid of confusing concepts like "home document" and "home sequential navigation focus order" which are definitions that appear mid-algorithm, and are likely not necessary in general
• Clean up implicit flag-like variables by making them more explicit enums
• Clarifying algorithm parameter types and return types
• Clean up wrapping
• Specifying that the "DOM" selection mechanism entails tree order
• Clean up recursive invocations of the sequential navigation search algorithm

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/interaction.html ( diff )

[domenic]

Big remaining question is tree order vs. shadow-including tree order. Otherwise, loving the cleanups and clarity improvements here.