Add TTML feature(s) for rw/rh <length> units

[spoeschel]

The rw and rh units can be used in <length> expressions and were added by TTML2. However they are currently not covered by a respective TTML feature - in contrast to the other allowed units of <length> (e.g. #length-em for the em unit).

[nigelmegitt]

I agree these should be added. TTML2's #length differs from TTML1's #length because of the changes to <length>, so there's an immediate problem to resolve. I think we need three things:

1. define a #length-rwrh (or similar) feature
2. define the TTML2 #length to be the same as the TTML1 #length by referencing the subset of features that were required in TTML1 only.
3. define a new #length-version-2 to include all the TTML2 <length>-related features.

[spoeschel]

Yes, this seems to cover it.

[nigelmegitt]

Note that at this time #739 does not add #length-version-2. Do we really need it?