Resizeable panels/windows

[Weird Constructor]

I envision a panel, sub window or window popup that I can grab on an edge and manually resize. It should be possible to make only certain edges resizeable so that some edges stay fixed.

Also here it would be good to have visual feedback with hovering on that edge and with the mouse cursor changing shape.

And here would also be a on_size_change callback good and a way to set the size via a message, to persist the size of the panel.