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.

Without this clicking trying to open a menu would imply this: