Support callbacks on different threads

[david-risney]

Great. Do these callbacks happen on the UI thread or on some other thread? I previously encountered problems with the iOS WebView because the OS required some things to happen in a callback in the UI thread, but I couldn’t get any data out of JavaScript because all the event handlers were also handled in the UI thread, but I would have to exit the callback I was currently in for the handlers to execute.

Originally posted by @my2iu in #90 (comment)

AB#28582327