Make clientManager easier to use

- Backend.clientManager renamed to Backend.clients
- clientManager is now a Mapping subclass

Before/After from Python:
backend.clientManager.clients[thing] → backend.clients[thing]

From QML:
Backend.clientManager.clients[thing] → Backend.clients.get(thing)

harmonyqml/components/Window.qml

@@ -8,7 +8,7 @@ ApplicationWindow {
     width: Math.min(Screen.width, 1152)
     height: Math.min(Screen.height, 768)
 
-    onClosing: Backend.clientManager.deleteAll()
+    onClosing: Backend.clients.removeAll()
 
     Loader {
         anchors.fill: parent