eframe: create multiple windows in sequence

[emilk]

Since #1889 we can now open an eframe window, close it, and keep running the application.

What we want to do is to then be able to open another window and run that for a while, etc.

Thanks to #1919 this now works on Windows, but this does not yet work on Mac, due to rust-windowing/winit#2431. Linux is unknown.

[emilk]

• Blocked on Can't create event loop ondemand rust-windowing/winit#2431

[winne42]

Linux is unknown.

@emilk Just ran examples/serial_windows on Ubuntu 22.10/Gnome. The second window opens up and then the program crashes immediately. That sounds like the behavior on Mac as described in rust-windowing/winit#2431.

[emilk]

• Update to winit 0.29 #3649 does not help on Mac :(