network: Handle cloned-mac setting

This can be set to 'preserve', 'permanent', 'random' or 'stable'. We
need to handle these values otherwise we can end up with the editor
being un-saveable.

Turn the entry in to a GtkComboBoxText with those items in it, allowing
a mac address to be typed too.

Partial copy of 85b6b659a140a59c3df787062e089a0b4e2a547d from
network-manager-applet.

panels/network/connection-editor/ce-page-ethernet.h

@@ -50,7 +50,7 @@ struct _CEPageEthernet
 
         GtkEntry        *name;
         GtkComboBoxText *device_mac;
-        GtkEntry        *cloned_mac;
+        GtkComboBoxText *cloned_mac;
         GtkSpinButton   *mtu;
         GtkWidget       *mtu_label;
 };