validate:launcher: Support mixed str/bytes control sequences.

validate:launcher: Support mixed str/bytes control sequences.

It is not safe for `_preformat_levels` to assume that all of the fields in a
`TerminalController` have the same type; at least in my environment, some of
these fields are populated with `bytes` while others remain strings.

This change conditionally applies decoding to each control sequence separately
using a helper function `_as_string`.  As a side-effect, it also eliminates some
code repetition in `_preformat_levels`.

Closes #50.