[ShellTrayIcon] add ShellTrayIcon, make ShellTrayManager use it

The actor emitted by ShellTrayManager is now ShellTrayIcon, a subclass
of ShellGtkEmbed which has several properties on it which are (or will
soon be) useful to the shell.

Part of the rearranging to use ShellTrayIcon means that we now show
the ShellEmbeddedWindow before creating its ShellGtkEmbed, which
requires a few modifications to ShellEmbeddedWindow (notably, telling
it at construct time what stage it will be drawn on, since it needs to
know that before it has a ShellGtkEmbed now).

https://bugzilla.gnome.org/show_bug.cgi?id=608869