[FIX] web: not possible to show a notification via client action

Before this commit, it was not possible to show a message using a client
action. (e.g. show a success message only with core in Python)

Now, it's possible using a client action. This client action call
displayNotification and so we can use the same options also.

A test is added to be sure that this action do not change the current action

Note: in the future a custom registry may be created in a refactoring
to specific function action registry.

X-original-commit: 8775fd670b6b3ec19a77fc0eab72d2223baa36c8

doc/reference/javascript_reference.rst

@@ -1213,6 +1213,10 @@ The notification system in Odoo is designed with the following components:
   destroy notifications whenever a request is done (with a custom_event). Note
   that the web client is a service provider.
 
+- a client action *display_notification*: this allows to trigger the display
+  of a notification from python (e.g. in the method called when the user
+  clicked on a button of type object).
+
 - two helper functions in *ServiceMixin*: *do_notify* and *do_warn*
 
 
@@ -1260,6 +1264,21 @@ Here are two examples on how to use these methods:
 
     this.do_warn(_t("Error"), _t("Filter name is required."));
 
+Here an example in python:
+
+.. code-block:: python
+
+    # note that we call _(string) on the text to make sure it is properly translated.
+    def show_notification(self):
+        return {
+            'type': 'ir.actions.client',
+            'tag': 'display_notification',
+            'params': {
+                'title': _('Success'),
+                'message': _('Your signature request has been sent.'),
+                'sticky': False,
+            }
+        }
 
 Systray
 =======