rose edit, rosie go: fix toolbar gtk warning.

This fixes the warning: Warning: invalid cast from GtkMisc'
  gtk.MenuToolButton.__init__(self, hbox, label)

which arises from using a hbox instead of a gtk.Misc object
for the iconwidget in the gtk.MenuToolButton. Just using
a gtk.Image instead of a gtk.HBox containing a gtk.Image
works fine.
Merge pull request #1151 from benfitzpatrick/rose-guis.fix-toolbar-gtk-warning

rose edit, rosie go: fix toolbar gtk warning.