[ttml2] add tts:position style

In order to address region anchoring as well as background image placement,
I've defined [1]:

   - <position> expression syntax [2]
   - tts:position style, applicable to tt, div, p, and region [3]

This property can be used instead of tts:origin on div and p (creating an
inline region) and on region in order to position a region within the root
container region.

Importantly, it can also be used on tt in order to position the root
container region with respect to a related media object region or
externally defined presentation region.

The same <position> syntax is used to position a background image with
respect to the padding rectangle of a region or content element's generated
areas. I'm attaching a small HTML test file (best viewed with Firefox)
showing a few examples of how it works with this context.

[1] https://dvcs.w3.org/hg/ttml/rev/272442265008
[2]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#style-value-position
[3]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#style-attribute-position