ttmlparse: don't filter out untimed whitespace

ttmlparse: don't filter out untimed whitespace

Significant whitespace in elements that don't have begin/end values
should inherit timing from its parent, or if no its parents have no
timing, from the document's Root Temporal Extent. Currently, such
whitespace is removed, which is not spec-compliant. Fix this by
retaining whitespace in content nodes, and assigning a Root Temporal
Extent of 24 hours to any significant whitespace whose parents have no
associated timing.

https://bugzilla.gnome.org/show_bug.cgi?id=781027