Control currently viewied event via RoomViewStore

Fix for https://github.com/vector-im/riot-web/issues/4224

Due to the way `MatrixChat` does a state update when the `view_room` dispatch fires and a second update when `RoomViewStore` sends an update, the current event ID and room ID were becoming out of sync. The solution devised was to have the event ID managed by the `RoomViewStore` itself and do any defaulting there (for when we revisit a room that we saved scroll state for previously).

This required a few changes:
 - The addition of `update_scroll_state` in `RoomViewStore` allows the `RoomView` to save scroll state for a room before swapping to another one. Previously the caching of scroll state was done in `RoomView`.
 - The `view_room` dispatch now accepts an `event_id`, which dictates which event is supposed to be scrolled to in the `MessagePanel` when a new room is viewed. It also accepts `event_offset`, but currently, this isn't passed in by a dispatch in the app, but it is clobbered when loading the default position when an `event_id` isn't specified. Finally, `highlighted` was added to distinguish whether the initial event being scrolled to is also highlighted. This flag is also used by `viewRoom` in `MatrixChat` in order to decide whether to `notifyNewScreen` with the specified `event_id`.

src/components/views/rooms/EventTile.js

@@ -381,6 +381,7 @@ module.exports = WithMatrixClient(React.createClass({
         dis.dispatch({
             action: 'view_room',
             event_id: this.props.mxEvent.getId(),
+            highlighted: true,
             room_id: this.props.mxEvent.getRoomId(),
         });
     },