Trim trailing whitespace from Dialogue lines early

**Not for merge yet.**

Match VSFilter. In particular, trailing `\N ` (with any non-zero number of spaces) no longer produces an empty line. VSFilter trims both trailing and leading whitespace early, but trimming leading whitespace late presents no such problem.

After #46 is fixed, I think `x->symbol == '\n'` can be removed from `IS_WHITESPACE` and the trailing whitespace trimming loop removed from `trim_whitespace`.