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