Fix accel char for '[x] Shadow' dialog checkbox
Speedup

* 1000x speedup for 200k tracked lines - just by reducing the progress
update frequency to 1upd/sec.
* Use Aegisub 3.0 subtitle API (delete & insert with multiple values) if
possible
* Insert new lines after the last originally selected line instead of
the first one.
Merge pull request #3 from tophf/master

Speedup & cosmetics