Remove type conversion functionality from Options

Automatically converting the type is problematic, because if we change
the type of an option later (imagine str -> list), it'll pass the old
value to the new type constructor before any upgrade hooks are run.
With 6ff1f53, there shouldn't be any need to convert the types anyway
because sip now does that for us.

The 'convert' functionality has been repurposed here for actual format
conversions. As it happens, the only Option that needs that is the
password field (rot13).
Convert QByteArray to unicode to fix exception