Rework startup and Application-Engine relation

- Application and Engine will be started by __init__.run() independently
- Exiting app will disconnect clients
- Signals like SIGINT (Ctrl-C) are now handled for proper exit

harmonyqml/components/Chat/Chat.qml

@@ -4,6 +4,7 @@ import "Banners"
 import "RoomEventList"
 
 HColumnLayout {
+    Component.onCompleted: Backend.pdb()
     property string userId: ""
     property string roomId: ""