Rough proof of concept of multiple windows in egui

Includes opening more windows at runtime and accessing / mutating state between windows. See https://youtu.be/hHL9riM5ELQ

Currently only tested on mac and windows.

Inspired by / derived from mini_fb_gl's multi_window example

known issues

if you open a bunch of popup windows and then close the root, the app might not exit and continues to run in the background.