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/Chat/Banners/InviteBanner.qml

@@ -30,16 +30,12 @@ Banner {
     buttonCallbacks: {
         "accept": function(button) {
             button.loading = true
-            Backend.clientManager.clients[chatPage.userId].joinRoom(
-                chatPage.roomId
-            )
+            Backend.clients.get(chatPage.userId).joinRoom(chatPage.roomId)
         },
 
         "decline": function(button) {
             button.loading = true
-            Backend.clientManager.clients[chatPage.userId].leaveRoom(
-                chatPage.roomId
-            )
+            Backend.clients.get(chatPage.userId).leaveRoom(chatPage.roomId)
         }
     }
 }

harmonyqml/components/Chat/Banners/LeftBanner.qml

@@ -22,9 +22,7 @@ Banner {
         "forget": function(button) {
             button.loading = true
             chatPage.canLoadPastEvents = false
-            Backend.clientManager.clients[chatPage.userId].forgetRoom(
-                chatPage.roomId
-            )
+            Backend.clients.get(chatPage.userId).forgetRoom(chatPage.roomId)
             pageStack.clear()
         },
     }