Event replacement API spans two classes

[KitsuneRal]

The API is spread across RoomMessageEvent and RoomEvent; the main reason for that seems to be that when the event is getting replaced, the data for the replacing event are put in unsigned and not in content JSON dictionary; but replacing an event with a non-message is not a good thing anyway. For now it makes sense to move isReplaced() and replacedBy() to RoomMessageEvent; and we'll see what MSC1849 will bring us.