Implement standalone bluetooth agent handler

[Simon Fels]

Currently the settings app registers an agent instance against BlueZ which can only be triggered when the application is currently active. To provide a better user experience we need a different place to implement the agent to have it running in the background all the time. indicator-bluetooth sounds like a suitable component for this but we need to sort out how we present the different user interface screens (trusted helper, notifications, units8, ....).