Previously when a client requests that a window should be docked the
shell would reparent the socket window onto the stage's window and
then use ClutterX11TexturePixmap to get a texture to represent the
window. This will not work if Clutter is no longer using the X11
winsys for example if it becomes its own display server. Instead this
patch leaves the socket window as a child of the root window and lets
mutter create a MetaWindow out of it. If Mutter is acting as a display
server then this mechanism will still work via the headless x server.

The ShellGtkEmbed instance now registers for notification of the
‘window-created’ signal of the display so that it can find the
MetaWindow that gets created to represent the socket window. When this
window is found it is prevented from being displayed on the screen by
setting the actor's opacity to 0. An input shape is then set on the
window to prevent it receiving any input.

Instead of being a ...