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 (closed).