Move tray implementation away from GtkSocket
From the main commit:
Reimplement tray icon support on top of NaXembed. NaTrayIcon now
becomes a subclass of this type, replacing GtkSocket. The
ShellTrayIcon wrapper now embeds directly a NaTrayIcon, the
ShellEmbeddedWindow GTK widget and ShellGtkEmbed ClutterClone
objects are no longer necessary to wrap a NaXembed socket window.