na-tray-child: Use absolute icon position as root event coordinates

na-tray-child: Use absolute icon position as root event coordinates

Currently we're using relative positioning when sending click events to
tray icon clients, and this leads to some apps (especially Qt ones) to
try to open the menus at such absolute coordinate under X11.

To prevent this to happen, let's get the root coordinate from the xembed
and let's use it to compute the synthetic event root x/y.

We could have even used the actual event position for this, but getting
it from the xembed makes this more consistent.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3283>