Introduce a RoomScrollStateStore

to keep the place we're scrolled to in rooms. This mainly eleimates
the extra, superfluous onRoomViewStoreUpdate callback that
happened when the previous room saved back its scroll state.
Moving the scroll state to a separate store means we can have this
not emit events because nothing needs to know when the scroll state
changes.

src/stores/RoomScrollStateStore.js

@@ -0,0 +1,37 @@
+/*
+Copyright 2017 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Stores where the user has scrolled to in each room
+ */
+class RoomScrollStateStore {
+    constructor() {
+        this._scrollStateMap = {};
+    }
+
+    getScrollState(roomId) {
+        return this._scrollStateMap[roomId];
+    }
+
+    setScrollState(roomId, scrollState) {
+        this._scrollStateMap[roomId] = scrollState;
+    }
+}
+
+if (global.mx_RoomScrollStateStore === undefined) {
+    global.mx_RoomScrollStateStore = new RoomScrollStateStore();
+}
+export default global.mx_RoomScrollStateStore;