gtk-embed: Hide window actors from pick

Since we started to support tray icons on wayland, the icon we show is not the actual XEmbed window. Instead, we let mutter create a MetaWindow for it, then use its window actor as source for a clone we (or nowadays: extensions) can add, remove and destroy freely.

To not let the real icon get in the way, we set an empty input shape and make its window actor fully transparent. This works OK on X11, but on wayland all events still go through Clutter, so any reactive surface actor inside the window actor will block events for any actors underneath (and status icons go into the top-windows group, so almost all actors are affected).

Luckily we can pile another hack onto the pile of status icon hacks ...

Fixes #191 (closed)