Remove use of attributes/queries for signature/status

[humdinger]

This is hrev33755.

At the moment Mail finds its signatures (stored by default in ~/config/settings/Mail/signatures) with a query for their _signature attribute.
Custom statuses(sp?) (stored by default in ~/config/settings/Mail/status) are of some binary format and have a _status attribute.

The use of attributes and queries for this is not very sensible. For once, those attributes have to be indexed, which they are at Mail's first start-up. Still, this can complicate restoring backups in a newly installed system.

Much easier for managing sigs and statuses is to use simple text files. File name is the sig-title the contents the sig-contents. The status is set by its file name alone.