buildEvent: Update the frame counts for events when edited in main .json file made by splitEvent

[Rarefoil]

I've checked, changing the frame counts in a single Scene is not saved. While the event is built, when you split it open with splitEvent, it shows the original number in the main .json file. Saved a scene as 0 or 1, it didn't matter. Saved a scene to 10 and adjusted the total amount of frames to compensate, didn't matter. This would be ridiculously helpful to have.

My SA Tools files were last updated on June 19th, but I don't believe buildEvent or splitEvent were updated since then.