Add the capability of transient windows.

[rodrigorc]

Description

In Win32 we have this: with_owner_window, that creates an owned top-level (overlapped in Win32 lingo) window that is actually owned by another: it is always drawn on top of its owner, and a few other platform rules.

But looking around I didn't find the equivalent code for other platforms. But I'm pretty sure that at least X11/MacOS have something more or less equivalent:

• X11 has XSetTransientForHint.
• MacOS has ordered:NSWindowAbove (or something...).

I think it should fall into the same category as WindowAttributes::with_position(), only available on some platforms. You can event argue that having a window owner is some kind of position.

Relevant platforms

No response

[jgcodes2020]

I will add that for Wayland, the equivalent is xdg_toplevel::set_parent()