Two-way Embedding External Native Windows

Being able to embed libui into projects by being able to convert managed window into a uiWindow (or any widget into a ui* widget) would be great. You are able to do this in other cross-platform UI toolkits such as [Qt](https://doc.qt.io/qt-5/qwindow.html#fromWinId), [GTK+](https://developer.gnome.org/gdk2/stable/gdk2-X-Window-System-Interaction.html#gdk-window-foreign-new), and [wxWidgets](https://docs.wxwidgets.org/trunk/classwx_native_window.html), so I feel as it might be feasible to ask for it in this toolkit.

Also being able to get the HWND (Win32)/FD (Unix) of any widget or element and pass that to usually a shared library so it can use it would be nice. Not sure if you can do this already, but there isn't any documentation and messing around with libui shows there is no way to go lower than a ui* component, correct me if I'm wrong though.