Add plumbing to send messages from the Online code to the JavaScript
code, and vice versa. Similar to what is done for iOS.
Sadly, it crashes. Multi-thread issues. Not surprisingly, it crashes
when I call webkit_web_view_run_javascript() in another thread than
the one where the GTK+ and other Webkit calls were done. I need to
come up with some clever way to do everything from the same thread.
(On iOS, I use dispatch_async(dispatch_get_main_queue(),...) to
schedule a block (i.e., a lambda expression) to be run in the main
thread.)
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