[FIX] web: handle widget order and custom sequence

An editable list can be drag-and-drop sorted by:

- displaying a sequence field,
- displaying a field with the `handle` widget

At first it was only working if the list was not specifically ordered,
but with 26bf35260 it also worked if the list was ordered on `sequence`
or `sequence ASC`.

This commit changes this so it works:

- if the list is not ordered (same as the original intention),
- if the list is ordered by the sequence field (widget `handle` or sequence)

This allows to have a custom model (on which we can't define _order) to be
ordered on a custom field.

This also remove a far-fetched oddity such as having the view ordered by
sequence but having a custom sequence field before `sequence`.

opw-704612

addons/web/static/src/js/views/list_view.js

@@ -1493,20 +1493,16 @@ ListView.Groups = Class.extend({
         });
     },
     setup_resequence_rows: function (list, dataset) {
+        var sequence_field = _(this.columns).findWhere({'widget': 'handle'});
+        var seqname = sequence_field ? sequence_field.name : 'sequence';
+
         // drag and drop enabled if list is not sorted (unless it is sorted by
-        // sequence (ASC)), and there is a visible column with @widget=handle
-        // or "sequence" column in the view.
-        if ((dataset.sort && dataset.sort() && dataset.sort() !== 'sequence'
-            && dataset.sort() !== 'sequence ASC')
-            || !_(this.columns).any(function (column) {
-                    return column.widget === 'handle'
-                        || column.name === 'sequence'; })) {
+        // its sequence field (ASC)), and there is a visible column with
+        // @widget=handle or "sequence" column in the view.
+        if ((dataset.sort && [seqname, seqname + 'ASC', ''].indexOf(dataset.sort()) === -1)
+            || !_(this.columns).findWhere({'name': seqname})) {
             return;
         }
-        var sequence_field = _(this.columns).find(function (c) {
-            return c.widget === 'handle';
-        });
-        var seqname = sequence_field ? sequence_field.name : 'sequence';
 
         // ondrop, move relevant record & fix sequences
         list.$current.sortable({