Add optional column on tree view

issue10989
review374321002

view/price_list_line_tree.xml

@@ -3,9 +3,9 @@
 this repository contains the full copyright notices and license terms. -->
 <tree>
     <field name="price_list" expand="1"/>
-    <field name="sequence"/>
+    <field name="sequence" optional="1"/>
     <field name="category" expand="1"/>
     <field name="product" expand="1"/>
     <field name="quantity"/>
-    <field name="formula" expand="1"/>
+    <field name="formula" expand="1" optional="0"/>
 </tree>

view/price_list_line_tree_sequence.xml

@@ -2,8 +2,9 @@
 <!-- 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 sequence="sequence">
+    <field name="price_list" expand="1"/>
     <field name="category" expand="1"/>
     <field name="product" expand="1"/>
     <field name="quantity"/>
-    <field name="formula" expand="1"/>
+    <field name="formula" expand="1" optional="0"/>
 </tree>

view/price_list_tree.xml

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