Tab navigation would be nice

After opening the preferences with Ctrl-p I cannot close the dialog.

Pressing Enter right away toggles the first option.
The cursor keys select different options, but "OK" / "Cancel" cannot be reached with it.
Tab and Shift-Tab seem to not have any impact.

I need to kill the python process to get out of it.

I've tried it with both Python 3.6 and 3.5.2, different terminals (urxvt, xterm, gnome-terminal), and also with different `display` settings (via config file).

Used version: v2017.1.1-4-gb38148e, but tried the release from PyPI first.
```
-e git+http://git.tiker.net/trees/pudb.git@b38148e6b05a88f5ff43a94925e8235ea025f3e4#egg=pudb
Pygments==2.2.0
urwid==1.3.1
```