refactor: Add sortOrderKey and check for this while Event creation

This makes it easier to write a store which just stores the Event json and
does not have a dedicated scheme for
each object like Moor has.

lib/src/event.dart

@@ -173,7 +173,7 @@ class Event extends MatrixEvent {
           : DateTime.now(),
       unsigned: unsigned,
       room: room,
-      sortOrder: sortOrder ?? 0.0,
+      sortOrder: sortOrder ?? unsigned.tryGet<num>(sortOrderKey) ?? 0.0,
     );
   }
 

lib/src/room.dart

@@ -54,6 +54,8 @@ const Map<HistoryVisibility, String> _historyVisibilityMap = {
 const String messageSendingStatusKey =
     'com.famedly.famedlysdk.message_sending_status';
 
+const String sortOrderKey = 'com.famedly.famedlysdk.sort_order';
+
 /// Represents a Matrix room.
 class Room {
   /// The full qualified Matrix ID for the room in the format '!localid:server.abc'.