Implement Screen Control Selection

[mikldt]

Each Player should have information about how to control its screen. For the purposes of this ticket we will only implement control method selection, but it should be implemented so more parameters may be added in the future.

Since this is likely to evolve, it might be best implemented as serialized JSON, as was done with the screen_on_off field. The field could be called screen_control_info.

The JSON should represent a hash. Right now there will only be one key, method. the associated value would be a string determining the control type, initially either dpms (default) or none.

The Player owner should be able to control this through the Edit Player view. This information should be available to the Player through the API.