quick_frontend, primitivus: mechanism to signal a new message reception when the window is hidden (even if the message is actually displayed):

- for now this is only used by Libervia

frontends/src/primitivus/primitivus

@@ -479,6 +479,13 @@ class PrimitivusApp(QuickApp, InputHistory):
         else:
             del self._early_popup
 
+    def isHidden(self):
+        """Tells if the frontend window is hidden.
+
+        @return bool
+        """
+        return False  # FIXME: implement when necessary
+
     def alert(self, title, message):
         """Shortcut method to create an alert message