erp5_direct_debit_payment: allow fulltext search on mandate's source reference

As source reference is the public reference of the Mandates (ie: RUM in the case of SEPA debits), it is even more useful than the simple reference

bt5/erp5_direct_debit_payment/PortalTypeTemplateItem/portal_types/Direct%20Debit%20Mandate.xml

@@ -59,6 +59,18 @@
             <key> <string>portal_type</string> </key>
             <value> <string>Base Type</string> </value>
         </item>
+        <item>
+            <key> <string>searchable_text_property_id</string> </key>
+            <value>
+              <tuple>
+                <string>title</string>
+                <string>description</string>
+                <string>reference</string>
+                <string>short_title</string>
+                <string>source_reference</string>
+              </tuple>
+            </value>
+        </item>
         <item>
             <key> <string>type_class</string> </key>
             <value> <string>Supply</string> </value>