Let VTTCue have a direct region member instead of a regionId string

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380
Remove the VTTRegion.track and .id attributes

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380

There are still uses of the region identifier concept to clean up,
in the parser and in getRegionById().
Remove the extensions to the TextTrack interface

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24380

With a direct association between cues and regions, the text track
itself doesn't need to know about regions, simplifying the API.
Call padding padding, not margin

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24329
Don't let regions modify the "initial containing block"

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24326

It's quite possible that regions rendering is now slightly broken,
but it was at least slightly broken before this commit also.
Make the Text Track Region(s) section plural to match Text Track Cues
Tweak the "parse a percentage" algorithm (editorial)

Returning IndexSizeError makes no sense. Saying that a number is
in a range of percentages also does not make sense.
Remove remaining percentage ranges

Numbers are not percentages.
Remove the comment about v2 feature requests

These and others are tracked with the "v2" keyword in Bugzilla.
Merge pull request #31 from foolip/regions-api

Don't extend TextTrack with WebVTT-specific attributes
Merge pull request #32 from foolip/padding-not-margin

Call padding padding, not margin
Merge pull request #33 from foolip/regions-initial-containing-block

Don't let regions modify the "initial containing block"
Merge pull request #34 from foolip/regions-plural

Make the Text Track Region(s) section plural to match Text Track Cues
Merge pull request #35 from foolip/percentage-parsing

Tweak the "parse a percentage" algorithm (editorial)
Merge pull request #36 from foolip/percentage-ranges

Remove remaining percentage ranges
Merge pull request #37 from foolip/v2-comment

Remove the comment about v2 feature requests
Simplify the handling of duplicate region ids
Merge pull request #38 from foolip/last-region

Simplify the handling of duplicate region ids
Add regions to "Collect WebVTT cue timings and settings" call

This was forgotten in commit 9a2f18a636bd47b6021ef8d94fc1d36aaf8eb2be.
Merge pull request #39 from foolip/using-regions

Add regions to "Collect WebVTT cue timings and settings" call