lib: index the content type of signature parts

lib: index the content type of signature parts

It's useful (*) to be able to easily find messages with certain types
of signatures. Having the mimetype: prefix searches fail for some
content types is also genuinely surprising (*). Index the content type
of signature parts.

While at it, switch to the gmime convenience constants for content and
signature part indexes.

*) At least for developers of email software!