Use urwid>=2.5.0 Scrollable and ScrollBar widgets (Fixes #244)

[rsekman]

The Scrollable and ScrollBar widgets implemented for stig were merged into urwid in 2.5.0. This PR updates stig to use the now native urwid widgets. This makes stig compatible with urwid>=2.5.0. However, this PR depends on https://github.com/urwid/urwid/pull/879 being merged and released. This is because stig tries to scroll widgets inside AttrMap in several places. After that bug is fixed, we'll need to bump the urwid requirement to whatever release it ends up in (likely 2.6.12), and then stig's version.