Allow Bevy windows to be spawned as a child of an existing window

## What problem does this solve or what need does it fill?

Currently, a Bevy app is designed to be a standalone application. Instead, I need to use a Bevy app within an existing application.

## What solution would you like?

I would like to be able to initialize a Bevy window as a child of an existing window; this seems like the most straightforward and promising way.

## Additional context

There was a previous discussion about this here #4838, which forked bevy_winit to create all windows as a child window using https://smithay.github.io/smithay/winit/window/struct.WindowBuilder.html#method.with_parent_window

It's unclear to me if this a parent window should be shared for all windows in the app, or per-window.

It seems winit child windows have poor multi-platform support (not sure if this matters?).

I'd love to implement this myself once I have some guidance on how to proceed.

Start with the previous discussion in #4838 and winit's WindowBuilder.with_parent_window reference, then determine whether the parent is shared or per-window and which platforms support child windows. Done means Bevy can initialize a window as a child of an existing application window with the intended ownership and platform scope documented.

Written by the indexing model from the issue text.