Replace cmd script with a new version written in Python.

- Fixes a bunch of bugs in the old Perl version, such as claiming
a command does not exist when it returned no output or only empty
lines (now an appropriate info message gets shown to the user in
that case), as well as generating faulty D-Bus calls when trying
to forward empty output lines to Konversation (output lines con-
taining only whitespace are no longer forwarded).
- stderr output is now shown to the user (not sent to the server,
of course) instead of being silently swallowed.
- Trailing whitespace is now stripped from output lines.