Mark last viewed message
There could be great to add a class to the last read message, to show it in a different color when entering in a room.
To do this we should add a new hashmap to AppOp, to store the last message id, related with the room, when the user changes room. And then, when we add messages again, we can mark that message with a new css class, then we can change the background color, add a border or add something to mark as different.
Other way to do this is to not use a class and add a new widget to the listbox with a line and a text in the middle, something like:
------- New messages ------