Implement a 3D widget

[Yamakaky]

The current Canvas widget supports only 2D graphics. It would be cool to have a 3D counterpart, maybe limited to wgpu at first. I see it basically as a boilerplate-free version of https://github.com/hecrj/iced/blob/0.1/examples/integration/src/main.rs. My personal use-case is to have a window with a 3D view on one side (only when the option is enabled) and the other side with configuration options.