frontends, core, plugin XEP-0045: leave implementation + better nick change

- memory: individual entity cache can be deleted
	- plugin XEP-0045: nick change are now detected and userChangedNick signal is sent instead of joined/left
	- plugin XEP-0045: leave implementation
	- frontends: userChangedNick signal management
	- Primitivus: an alert is shown in notification bar in case of error in sendMessage

src/memory/memory.py

@@ -750,6 +750,18 @@ class Memory:
                 ret[key] = entity_data[key]
         return ret
 
+    def delEntityCache(self, entity_jid, profile_key):
+        """Remove cached data for entity
+        @param entity_jid: JID of the entity
+        """
+        profile = self.getProfileName(profile_key)
+        try:
+            del self.entitiesCache[profile][entity_jid.userhost()]
+        except KeyError:
+            pass
+
+
+
     def addWaitingSub(self, _type, entity_jid, profile_key):
         """Called when a subcription request is received"""
         profile = self.getProfileName(profile_key)