Allow to get a list of visible embedded Windows

[YeldhamDev]

While it's currently possible to manipulate Windows that are native (via DisplayServer.window_*() methods), there are no ways to get which embedded windows are being used. This PR creates the get_embedded_subwindows() method for the Viewport, that lists all visible embedded windows tied to one.

[Sauermann]

Implementation looks correct. Also I have tested, that the functionality works.
I would probably move the code to the other subwindow-functions, like Viewport::is_embedding_subwindows.
Can't say anything about the topic, if the functionality is needed.

[YeldhamDev]

@Sauermann

I would probably move the code to the other subwindow-functions, like Viewport::is_embedding_subwindows.

Good idea, I've done so.

[akien-mga]

Thanks!