Tracing `kara_split`

[0tkl]

Among the icons in src/bitmaps/icon/, there are two icons that do not exist in any of the current code: kara_split and forums_button.

https://github.com/TypesettingTools/Aegisub/blob/bcd7a132aa87b5e62d36b4936ce19157d72dc59d/src/bitmaps/button/kara_split_64.png

https://github.com/TypesettingTools/Aegisub/blob/bcd7a132aa87b5e62d36b4936ce19157d72dc59d/src/bitmaps/button/forums_button_64.png

It's easy to find that the usage of forums_button was removed in 2018: 2f500cd

However, I couldn't find any record of kara_split being used in the repo's history. I suspect this image was simply used as a background for kara_split_accept and kara_split_cancel during the design process, or perhaps just my search keywords were off-target. If anyone finds relevant information, please let me know so I can add it to the commit message, see #496.