rose edit, rosie go: fix toolbar gtk warning.

When loading `rose edit` or `rosie go` with the `--debug` switch,
using a GTK 2.20+ version, they produce the following warning:
```
Warning: invalid cast from `GtkHBox' to `GtkMisc'
  gtk.MenuToolButton.__init__(self, hbox, label)
...
GtkWarning: IA__gtk_misc_set_alignment: assertion `GTK_IS_MISC (misc)' failed
```

which arises from using a hbox instead of a `gtk.Misc` object
for the iconwidget in a `gtk.MenuToolButton`. Just using
a `gtk.Image` works fine.

@matthewrmshin, please review.