Add multiwindow support

[ecton]

No description provided.

[dagit]

Having used a few other rust gui libraries and bumped into this limitation, I would say add this support as early as you can. It is likely to impact your API in ways that are hard to foresee.

Also, yes please. Multiwindows is important to me.

[ecton]

I don't have a convenient way to link a gif/mp4 into a github issue, so I've shared a demo gif on my Mastodon. The example that's being shown is examples/multi-window.rs

This wasn't too difficult for me to get working today because I had planned for it from the beginning.