Make the vtables less absurdly huge (knocks 100KB off aegisub32.exe),
eliminate some copies of the values when constructing the options, and
use an enum class for the value type.
The grid needs to be able to map AssDialogue * to a row number, and just
storing it in the AssDialogue is nearly strictly better than a std::map
mapping events to rows. Probably will be of use elsewhere as well.
