[FIX] web: fix date picker fields do not capture the focus event when invisible

Before this fix, in form view, when a datepicker was not visible (because
they were on another page or literally invisible), when the user pressed
TAB and the datepicker was positionned after the current field in the
form, it was capturing the focus during the activate phase of the
navigation_move.
This is not correct for invisible elements.

After this fix, only focusable datepickers get the focus during the
activation phase of the navigation_move.

co-author: aab-odoo (aab@odoo.com)

addons/web/static/src/js/fields/basic_fields.js

@@ -538,7 +538,7 @@ var FieldDate = InputField.extend({
      * @override
      */
     activate: function () {
-        if (this.datewidget) {
+        if (this.isFocusable() && this.datewidget) {
             this.datewidget.focus();
             return true;
         }