embedded-window: Set as app-paintable to workaround opaque region issues

In specific cases, GTK+ does not have enough information to set a correct
opaque region, in which the recommended fix is to set your window as
app-paintable. In the tray icon case, the socket window was considered
opaque but GTK+ as it had a solid window background, but it cannot have an
opaque region set, as the plug isn't composited against the socket, but
instead punches through the socket window.

https://bugzilla.gnome.org/show_bug.cgi?id=707614