[ttml10se] add appendix on time expression semantics

I've posted an update to TTML1.0 SE at [1], containing a new appendix (H)
on time expression semantics in order to resolve my action item [2] on
issue [3]. Although I have addressed the time expression semantics for
clock [4] and media [5] time bases, I am still working on the spec text for
the smpte time base [6]. I'm doing this in two phases in order to give the
group the most opportunity for review. Please give this new text a thorough
review, as it is marked as normative.

I have also added error conditions if frame or sub-frame terms are used
with time expressions when clock time base applies. See last two paragraphs
of [7].

Finally, I have started updating text elsewhere to make reference to this
new appendix as appropriate [8].

For relevant change sets, see [9][10].

[1]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8
[2] https://www.w3.org/AudioVideo/TT/tracker/actions/140
[3] https://www.w3.org/AudioVideo/TT/tracker/issues/199
[4]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8#time-expression-semantics-clock
[5]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8#time-expression-semantics-media
[6]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8#time-expression-semantics-smpte
[7]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8#timing-value-timeExpression
[8]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10/spec/ttaf1-dfxp.html?content-type=text/html%3bcharset=utf-8#timing-time-intervals
[9] https://dvcs.w3.org/hg/ttml/rev/2f3da27f0266
[10] https://dvcs.w3.org/hg/ttml/rev/ac3cb6491db5