Add Gtk sample

[ysbaddaden]

Creates a blank Gtk application and window that runs the UI in its own dedicated thread, without blocking the main thread (be it MT or ST).

[ysbaddaden]

The sample app could showcase the Gtk context and default context communication, and show how the UI keeps reacting. Like a text entry where we can enter a URL, that will spawn a fiber to fetch it, with a spinner (something to show that the UI don't block) then update the UI with some stats, like the response status, content type, content size, ...