QtNfc API improvements

As we have the unique opportunity to break and improve the QtNfc API, here are some comments.

Here's an example of how I used the API on Harmattan: http://gitorious.org/xbmcremote/xbmcremote/blobs/master/apps/harmattan/nfchandler.cpp

The main points are:

• The error() does not seem to support enough information (which might be a backend or even specification issue tho). For example, writes fail if the payload is too big for the tag size. If there is the option to write less data, its basically a try and error. Would be nice to know if the write failed because of this, or a possibility to check the tags size in advance.

• The ndefMessageWritten() signal does not contain any status information. This forces the API user to manually check if error() has been emitted and keep track himself about it.