Extend QML NearField item to support neard

The property 'polling' has been added to query the current state
and to allow the user to start and stop target detection. The
signals 'tagFound' and 'tagRemoved' have been added to allow
the user to react to those events.

Change-Id: Ieb07b5b799679982429a66f1790fa9985c55a24d
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>