Add optional column on tree view

issue10989
review374321002

view/carrier_tree.xml

@@ -3,6 +3,6 @@
 this repository contains the full copyright notices and license terms. -->
 <tree>
     <field name="party" expand="1"/>
-    <field name="carrier_product" expand="1"/>
-    <field name="carrier_cost_method"/>
+    <field name="carrier_product" expand="1" optional="0"/>
+    <field name="carrier_cost_method" optional="1"/>
 </tree>

view/selection_list.xml

@@ -2,7 +2,7 @@
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
 <tree>
-    <field name="from_country" expand="1"/>
+    <field name="from_country" expand="1" optional="0"/>
     <field name="to_country" expand="1"/>
     <field name="carrier" expand="2"/>
 </tree>

view/selection_list_sequence.xml

@@ -2,7 +2,7 @@
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
 <tree editable="1" sequence="sequence">
-    <field name="from_country" expand="1"/>
+    <field name="from_country" expand="1" optional="0"/>
     <field name="to_country" expand="1"/>
     <field name="carrier" expand="2"/>
 </tree>