Progress Bar for Unread Messages

[plugyawn]

Possibly addressed by #897.

The current system doesn't have any scroll slider to show how much you'll have to scroll down to read all messages in the stream/topic. We could use urwid's ProgressBar to show it, perhaps?

[neiljp]

This is specifically regarding unread messages, so does come under the remit of #897 - which is very broad.

Regarding a scrolling mechanism (or at least a marker), that can work at a representative level but not necessarily an accurate 'message height' level like a traditional scrollbar - we don't know how long (how many lines) each unread message is.

Depending where this is located this could be confusing, since this would only be unreads, not general messages in a narrow. Would this be a percentage of all messages in a narrow? We don't necessarily have that information, and as we read more messages we just know that the number goes down.

A general scrollbar could be used, but it has its own drawbacks related to the incomplete message information a client generally has.