Generalize InvitedRoom/JoinedRoom/LeftRoom (in _SyncData)

[yyportne]

_SyncData contains the types InvitedRoom, JoinedRoom and LeftRoom. Currently they don't have a generalized type. This leads to code duplication and in some cases triplication.

All three types contain synchronisation data for a specific room. Common properties are the room ID #getId() and the state events #getState().

A generalized type RoomData (or some other name) could be

interface RoomData {
   String getId(); // room ID
   State getState(); // state events
}

For better distinction between the RoomData subtypes add a RoomDataKind enum

enum RoomDataKind {
  JOINED,
  INVITED,
  LEFT
  ;
}
interface RoomData {
  …
  RoomDataKind getKind();
}