Accounts data is now stored through qtkeychain. This allows easier deployments
on platforms where KWallet is not available or desiderable.

qtkeychain's API is less expressive than KWallet's, so this port resulted in a bit more LOCs:

* it's not possible to store a QMap, we need to serialize it to QByteArray first.
* it's not possible to retrieve the list of stored entries (accounts), so we store an
  additional entry with the list of all stored accounts.