Started making changes for redesigning the positioning mechanism:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20037

This change adapts the data model (section 3.1)
and the syntax description (section 4.4.2).
The latter mainly has changes of the notes.
Reorganise the paragraphs in the rendering section a bit to allow easier reading.
Update the rendering section to support the positioning mechanism proposed
in https://www.w3.org/Bugs/Public/show_bug.cgi?id=20037#c15 for text positioning.
Adding overflow:hidden to cue background box.
Restore the .text attribute on VTTCue, since the attribute has
been removed on the parent interface, see
https://github.com/w3c/html/commit/580e7e6f98fbd82586aa7e894efbe8c16b99701f

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22905
Adding a static version of the spec, since respec is not reliable.
Also pointing to Overview.html now as the editor draft file.
Re-use common respec on source file - it got fixed.
Move to respec 3.2
Remove extraCSS.
Mark bug-assist as remove after spec creation.
Fix 2 typos found by respec 3.2.
Move local respec to 3.2.4 too.
Make "text position" actually define the position of the cue box as
discussed in https://www.w3.org/Bugs/Public/show_bug.cgi?id=20037 .
Remove surplus "computed text position anchor point".
Started redesigning the box positioning mechanism.
This patch introduces an example and a work-over of the data model.
Syntax and Parser to come.
Oops, fix the example to actually use the correct align values.
Add link to previous draft.
Tidy up references to WHATWG spec a bit.
Bring back a SOTD section, because ReSpec requires it.
Specify syntax for "WebVTT line position cue setting".
Also rename "cue position alignment" to "text position alignment" to
stay in sync with the "text position" cue setting.
Specify text position alignment syntax.
Set default values for cue settings in parsing.
Fix up parsing of line setting.