change timeline hierarchy, as prep to split timeline in sections at thread boundaries
before hierarchy was RoomView > TimelinePanel > MessagePanel > ScrollPanel > *EventTile
now it's RoomView > TimelineNavigationPanel > ScrollPanel > NonThreadedSection > TimelinePanel > MessagePanel > *EventTile
there is still only one section, as I'm trying to maintain the old functionality with the new view hierarchy.
jumping to locations in the timeline, sending read receipts, read markers are all completely broken,
but you should be able to view the timeline and paginate.