adding QtQuick.Dialogs, with FileDialog implementation

This will obsolete the QFileDialogItem in desktop components and be
available to all QtQuick applications.  The QML FileDialog type is
dynamically defined in the plugin by detecting which implementation
will work on the current platform.

Change-Id: I073c7a84bff6c02cf592dc46822a5a4b9c9bcaea
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>