Fix using negative numbers for startFrame.
Hook up relative checkbox.

This is a behavioral change. There's no real reason to have it, but it
was requested, and easy enough to do.
Use processed config for stripping fields from motion data.
Stop collecting headers when dialog lines are encountered.

Saves time iterating through the entire script. Also, allegedly Aegisub
3.2 guarantees header lines (info, styles) will come before the
dialogue lines.
Fix linear tracking.

I somehow managed to completely neglect to test it at all.
Version bump.