[12.0] web_notify: improve popup UI (#1231)

* [ADD]: all available bootstrap notifications (success/danger/warning/info/default)
* [IMP] use black color for text for default notification.
* [FIX] reverted require string for `bus.Longpolling` and rename `on_message_received` to `on_message` to prevent collisions.

web_notify/readme/USAGE.rst

@@ -2,14 +2,32 @@
 To send a notification to the user you just need to call one of the new methods defined on res.users:
 
 .. code-block:: python
-  
-   self.env.user.notify_info(message='My information message')
 
-or 
+   self.env.user.notify_success(message='My success message')
+
+or
 
 .. code-block:: python
-  
-   self.env.user.notify_warning(message='My marning message')
+
+   self.env.user.notify_danger(message='My danger message')
+
+or
+
+.. code-block:: python
+
+   self.env.user.notify_warning(message='My warning message')
+
+or
+
+.. code-block:: python
+
+   self.env.user.notify_info(message='My information message')
+
+or
+
+.. code-block:: python
+
+   self.env.user.notify_default(message='My default message')
 
 .. figure:: static/description/notifications_screenshot.png
    :scale: 80 %