tray/na-xembed: Use XShape to remove input on socket window

tray/na-xembed: Use XShape to remove input on socket window

We don't want this window to take any input as, a part from blocking the
events on the shell representation itself, they may potentially make the
plugged window to do anything with the event is processing (such as
showing tooltips). And we don't want this since we only want to send to
those windows only the synthetic events that we explicitly control.

This was already the case before of commit ab60902058 but a similar
approach was not replicated, leading to input events being fully
processed from tray icons plugs.

It requires adding an explicit build dependency on Xext, but this is
something that mutter already depends on so not really a new dependency

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7613
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3818>