TTML - Implement style composition

Our TTML implementation doesn't implement styling correctly. For instance, in this sample:

<p xml:id="subtitle1" region="bottom" begin="00:00:10.000" end="00:00:20.000" style="paragraphStyle">
	<span style="spanStyle">One line Subtitle.</span>
</p>

We only account for the "paragraphStyle", but leave the "spanStyle" aside.

added Component::Subtitles Severity::normal Type::feature Version::master git + 1 deleted label

changed milestone to %Features paradize

Added attachment:

• backgroundColor-region-p-span-001-ttml.xml

d86da185

added Status::fixed label

closed

changed milestone to %3.0 FF

Milestone renamed