Move list layout management to its own store

This is more general maintenance than performance as the RoomList doesn't need to be generating layouts for the sublists, and it certainly doesn't need to be creating a bunch of extra ones. 

The sublists are perfectly capable of getting their own layout instance and using it, and we are perfectly able to limit the number of these things we create through the session's lifespan.

src/@types/global.d.ts

@@ -21,6 +21,7 @@ import ToastStore from "../stores/ToastStore";
 import DeviceListener from "../DeviceListener";
 import { RoomListStore2 } from "../stores/room-list/RoomListStore2";
 import { PlatformPeg } from "../PlatformPeg";
+import RoomListLayoutStore from "../stores/room-list/RoomListLayoutStore";
 
 declare global {
     interface Window {
@@ -34,6 +35,7 @@ declare global {
         mx_ToastStore: ToastStore;
         mx_DeviceListener: DeviceListener;
         mx_RoomListStore2: RoomListStore2;
+        mx_RoomListLayoutStore: RoomListLayoutStore;
         mxPlatformPeg: PlatformPeg;
     }