Use popupParent as X11 parent window (#14366)

Previously, the popupParent was ignored when present causing popups to
have the X root window as its parent.

This cause issues on gamescope (SteamOS session compositor) where that window would end up behind the
main window.

Signed-off-by: Mary Guillemard <mary@mary.zone>
Co-authored-by: Max Katz <maxkatz6@outlook.com>

src/Avalonia.X11/X11Window.cs

@@ -142,7 +142,9 @@ namespace Avalonia.X11
             defaultWidth = Math.Max(defaultWidth, 300);
             defaultHeight = Math.Max(defaultHeight, 200);
 
-            _handle = XCreateWindow(_x11.Display, _x11.RootWindow, 10, 10, defaultWidth, defaultHeight, 0,
+            var parentHandle = popupParent != null ? ((X11Window)popupParent)._handle : _x11.RootWindow;
+
+            _handle = XCreateWindow(_x11.Display, parentHandle, 10, 10, defaultWidth, defaultHeight, 0,
                 depth,
                 (int)CreateWindowArgs.InputOutput, 
                 visual,