Support for writeMap for kwallet

[CarlSchwan]

KWallet allows for some reasons to store maps. This pause a problem when wanting to migrate away from kwallet to qtkeychain, since the data stored in that format is not accessible.

Would you be open to adding a writeMap and a readMap method to QtKeychain. On the kwallet backend, this would just use kwallet stuff and on the other backends, this would either do multiple save jobs for each keys or convert the QMap to a QByteArray and then save that.

I would be happy to implement it once I get the green light :)