Use selection widget for journal fields

It is expected to have only a small number of journals.
But also it is uncommon to create of a new journal from a form.
So it provides a better user experience to use a selection widget.

modules/account_dunning_fee/view/dunning_fee_form.xml

@@ -9,7 +9,7 @@ this repository contains the full copyright notices and license terms. -->
     <label name="product"/>
     <field name="product"/>
     <label name="journal"/>
-    <field name="journal"/>
+    <field name="journal" widget="selection"/>
     <label name="compute_method"/>
     <field name="compute_method"/>
     <newline/>