Alternative to WinHandler::connect()

[lord]

Writing down this idea just so I don't forget: one thing I've noticed is most implementations of WinHandler end up containing a ton of Options, which only get filled in after connect() is called. If we changed set_handler to take a FnOnce(&WindowHandle) -> WinHandler as an argument instead of an explicit WinHandler, perhaps we could handle some complexity for the user? (Although the user must be careful to not call any methods of the WindowHandle during initialization that would require the WinHandler to exist.)