This also closes bug #713808, as lesser-used fields (Reply-To, Bcc)
are hidden unless the user expands the composer to show them. Right
now that is an option in the composer's toolbar menu; that may change
in the future if we can find the right place to put the widgetry.
N.B. The actions for the menu need to be set through the UIManager, not
through the ActionGroup. Otherwise you get warnings about
gtk_accel_label_set_accel_closure.
