Speedup & cosmetics

* 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 - the new implementation is assuming the original selection was contiguous, which is quite probable in fact due to the only custom sort method available is 'by time', meaning the user explicitly/obsessively wants the lines to be grouped in contiguous blocks per each frame/framespan, so we can easily expect he/she has arranged the original lines in a contiguous & sorted by time block as well.

* Fix accel char for [x] Shadow dialog checkbox.