This commit adds a scrollbar that shows the scroll position but
that does not currently allow dragging to scroll.
The scrollbar occupies the right hand side window padding.
The width of the scrollbar can be set by explicitly configuring
`window_padding.right`. If the right padding is set to 0 (which
is its default value), and the scroll bar is enabled then the cell
width will be used for the right padding value instead.
The scrollbar can be enabled/disabled via this config setting:
```
enable_scroll_bar = true
```
Its color by this:
```
[colors]
scrollbar_thumb = "#444444"
```
(Note that color palette config will be reloaded when the config
file is changed, but you'll need to spawn a new tab/window to
see the effects because we cannot assume that a config reload should
always replace a potentially dynamically adjusted color scheme in
a tab).
