fix file panel scroll position lost on room change

src/stores/RoomScrollStateStore.js

@@ -33,6 +33,7 @@ class RoomScrollStateStore {
         //    pixelOffset: the number of pixels the window is scrolled down
         //        from the focussedEvent.
         this._RoomViewScrollStateMap = {};
+        this._FilePanelScrollStateMap = {};
     }
 
     getRoomViewScrollState(roomId) {
@@ -42,6 +43,14 @@ class RoomScrollStateStore {
     setRoomViewScrollState(roomId, scrollState) {
         this._RoomViewScrollStateMap[roomId] = scrollState;
     }
+
+    getFilePanelScrollState(roomId, scrollState) {
+        return this._FilePanelScrollStateMap[roomId];
+    }
+
+    setFilePanelScrollState(roomId, scrollState) {
+        this._FilePanelScrollStateMap[roomId] = scrollState;
+    }
 }
 
 if (global.mx_RoomScrollStateStore === undefined) {