Remove unused variable
Move character count stuff to libaegisub
Use ICU directly for character counting

ICU docs say not to create a new break iterator each time as
boost.locale does, and in fact creating the break iterator is about 90%
of the run time of the character counter, so use ICU directly and cache
the break iterator.
Update the character counter synchronously

It's not longer even vaguely in the realm of slow enough to justify
dumping off on a background thread.
Don't repopulate the actor/effect fields when the active line changes

Speeds up switching lines on a totally arbitrary test file that doesn't
even use very many actors or effects by ~20%.
Add an option to ignore punctuation for the character counter