Don't use ClutterX11TexturePixmap directly to embed docked windows

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 subclass of ClutterX11TexturePixmap, ShellGtkEmbed
is now a subclass of ClutterClone. When the MetaWindow is found for
the socket window the clone's source is set to the invisible actor for
the window so it can be displayed in the panel as before.

The ShellEmbeddedWindow no longer needs to know what the stage is
because it no longer reparents the socket window. Therefore the
ShellTrayManager doesn't need to know the stage either so
shell_tray_manager_manage_stage has been replaced with just
shell_tray_manager_manage_screen.

https://bugzilla.gnome.org/show_bug.cgi?id=693438