Add trailing slash to directories in `bzr ls` output

`bzr ls` should add a trailing slash to directories, either by default or via a new option, to differentiate them from files. The verbose mode does it, but it also adds information about versioning. The trailing slash should also be available without that version information.

For now, on POSIX platforms, this does the trick:
`bzr ls -v -V | cut -c 10-`

Thanks to jml, fullermd, and spiv in #bzr for participating in the discussion on this.