Switch TUI to urwid

[thw26]

https://github.com/urwid/urwid

The current TUI is in progress of creating a high level curses/dialog API. Switching to urwid would let us drop tui_screen.py, tui_curses.py, and tui_dialog.py, and it would also let us drastically simplify tui_app.py and likely launching the TUI app from main.py.

This would in turn reduce our custom code maintenance but introduce another project as a dependency.

I believe this transition would be worth the effort, particularly surrounding our desires in #147 and in #206.

[thw26]

My tests so far have much functionality in place, except for choosing the install directory. This will require more work on my part to get right.