Convert RoomView to using a TimelineWindow

Instead of using the Room's active timeline directly, use a
TimelineWindow. This shouldn't (yet) have much effect, beyond maybe making
scrollback after a gappy sync slightly more efficient.

For now, I have disabled the 'restoreScrollState' functionality. This will be
reinstated once I land the link-to-event code.

src/MatrixClientPeg.js

@@ -39,7 +39,8 @@ function createClient(hs_url, is_url, user_id, access_token, guestAccess) {
         baseUrl: hs_url,
         idBaseUrl: is_url,
         accessToken: access_token,
-        userId: user_id
+        userId: user_id,
+        timelineSupport: true,
     };
 
     if (localStorage) {