Link VTTRegion to the interface definition
Link to the <audio> and <video> elements
Drop the "dom-" prefix to interface/attribute/function definitions

ReSpec still prefixes them with "dfn-" and they aren't needed to make
the IDs unique.
Fix the links for the VTTCue and VTTRegion constructors
Fix the links to attributes in tables

This includes a typo fix: direction -> vertical
Merge pull request #80 from foolip/xref

Fix a number of cross-reference issues