Bracketed or parenthesized alternative KeySignature

Sometimes bracketed or parenthesized KeySignatures indicate an alternative key.
The here proposed alternativeKey-function tries to do so.
It takes three arguments: the tonic-pitch of the original key, the tonic-pitch of the alternative key and the scale. An optional argument is provided as well. It determines whether to use brackets or parentheses, possible settings are 'bracketified (default) or 'parenthesized. Other settings will issue a warning and the alternative KeySignature will be printed without any brackets or parentheses.
Limitations
- works only for minor and major, other scales are not supported
- it's not possible to use a different scale for the alternative key