This changes the Windows text handling so that keyboard events are sent to the framework first for handling, and then passed to the text input plugin, so that the framework has a chance to handle keys before they get given to the text field.

This is complicated by the async nature of the interaction with the framework, since Windows wants a synchronous response. So, in this change, I always tell Windows that the event was handled, and if the framework (eventually) responds that it wasn't, then I synthesize a new event and send it with SendEvent.

I also added support for detecting "extended" keys, since that was missing, and converted the OnKey handlers in the API to return a bool to indicate whether or not they have handled the event.