[IMP] ir_ui_view: Log warning on incorrect 'groups' attribute

Purpose
=======

They are a lot of places where we set an attribute 'groups' on a view element and:

- The group doesn't exist anymore
- The group xmlid is not correct
- The group xmlid exists but the modularity is not respected (example: group_stock_user
  used in a view in the 'product' module).

Where it happens, nothing warns the user or the developer. The element is just never
rendered.

Specification
=============

In the method _check_xml, log a warning if a 'groups' attribute is set and if
a group doesn't not exist at that moment.

odoo/addons/base/models/ir_ui_view.py

@@ -320,6 +320,12 @@ actual arch.
                         self.raise_view_error(message, self.id)
         return True
 
+    def _check_groups_validity(self, view, view_name):
+        for node in view.xpath('//*[@groups]'):
+            for group in node.get('groups').replace('!', '').split(','):
+                if not self.env.ref(group.strip(), raise_if_not_found=False):
+                    _logger.warning("The group %s defined in view %s does not exist!", group, view_name)
+
     @api.constrains('arch_db')
     def _check_xml(self):
         # Sanity checks: the view should not break anything upon rendering!
@@ -332,6 +338,7 @@ actual arch.
             view_arch_utf8 = view_def['arch']
             if view.type != 'qweb':
                 view_doc = etree.fromstring(view_arch_utf8)
+                self._check_groups_validity(view_doc, view.name)
                 # verify that all fields used are valid, etc.
                 self.postprocess_and_fields(view.model, view_doc, view.id)
                 # RNG-based validation is not possible anymore with 7.0 forms