[MIG] web_m2x_options_manager: Migration to 15.0

- Fix postprocess inherit (removed from v15), and use _postprocess_tag_field
for create/edit options: In v14 it worked with inheriting the "postprocess"
method because in each recursive call it passed a section of the view, in the
way that it went through all the child nodes of the root node, and passed through
the fields, and now in v15 the method "_postprocess_view "it just does the while,
and we never go through the nodes.

- Fix pylint errors: xml-deprecated-tree-attributte, attribute-string-redundant,
translation-positional-used

web_m2x_options_manager/views/ir_model.xml

@@ -17,7 +17,7 @@
                         nolabel="1"
                         context="{'default_model_name': model}"
                     >
-                        <tree string="Fields Description" editable="bottom">
+                        <tree editable="bottom">
                             <field name="model_name" invisible="1" />
                             <field
                                 name="field_id"