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/Window.qml

@@ -8,6 +8,8 @@ ApplicationWindow {
     width: Math.min(Screen.width, 1152)
     height: Math.min(Screen.height, 768)
 
+    onClosing: Backend.clientManager.deleteAll()
+
     Loader {
         anchors.fill: parent
         source: "UI.qml"