[WIP] Add navigation buttons below messages

[alya]

We want to add navigation buttons below the bottom message in the view. In addition to the buttons being useful themselves, by allowing the focus rectangle to be on these buttons, we'll be able to change the marking as read algorithm to only mark the messages above the focus rectangle as read.

The main questions are:

1. Which buttons we want?
2. Do we want to show hot keys? I
3. What are the levels of attention on those buttons? The screenshots below have 2 levels of neutral intent (low and medium)... but we could also utilize intent colors (brand, info, warning, danger)

This issue is not ready to be worked on.