Separate UI state info from functional ASS info headers a bit

Prefix script info entries that are just storing Aegisub UI state with
"Aegisub ", and use a separate AssFile method to get/set them.
Make saving transient UI state in the file optional

It significantly increases the amount of noise when tracking subtitle
files in source control.

Closes #1535.
Allow using , as the separator in time edit controls
Build wx with wxStandardPaths enabled

It's still needed to get the path to the executable when restarting to
switch UI languages.
Fix crash when the video height or width is zero

Closes #1555. Closes #1644.
Move libass fork to Aegisub org on github