Bring back functionatity to register smarphone sensors to the MBP

[ana-silva]

By pressing the button + to add a new device, a list of sensors to be selected are shown. However, nothing happens by selecting Register Smartphone and Sensors.

• reactivate smartphone registration functionality
• update the style dialog header text 'Smartphone sensors:' so that it becomes visible, e.g., by using bold, blue or bigger letters