API additions for the Plasma client

This patch contains a few (source-compatible) API additions needed for
the Plasma client.

* return QNetworkReply* to caller for tracking status and error of
  requests such as mkdir, getWebDAVPath and getRequest
* Add a setter for the QNetworkAccessManager. This allows us to route at
  least some of the network requests through KIO in the Plasma client
* Add a setter for the remotePollInterval. This should be enough API to
* make it possible to adapt the polling interval to the client's machine
  state, e.g. sync less often on battery, or somesuch