Allow DDC brightness control through other applications

[samedwards]

It looks like the current implementation uses an XPC service for setting/getting the brightness, contrast, list of displays, and the ability to "flip" the display.

From what I can see, right now only the Manager application and Elgato's CameraHub application seem to be whitelisted to communicate with the XPC service, so I would imagine that there will need to be some method to get other applications authorized for this service

$ strings DisplayLink\ Manager.app//Contents/MacOS/DisplayLinkXpcService
[...]
DlmXpc::Service: Processing command to return list of displays
DlmXpc::Service: Processing command to %{public}@ display %{public}@
DlmXpc::Service: Processing command to set contrast to %{public}.2f for display %{public}@
DlmXpc::Service: Processing command to set brightness to %{public}.2f for display %{public}@
DlmXpc::Service: Processing command to set flip setting to %{public}@ for display %{public}@
DlmXpc::Service: Notifying clients that contrast of display %{public}@ was changed to %{public}.2f
DlmXpc::Service: Error associated with client: %{public}@
DlmXpc::Service: Notifying clients that brightness of display %{public}@ was changed to %{public}.2f
DlmXpc::Service: Notifying clients that flip setting of display %{public}@ was changed to %{public}@
DlmXpc::Service: Notifying clients that displays have changed. New displays list:
[...]
certificate leaf[subject.OU] = "73YQY62QM3"
certificate leaf[subject.OU] = "Y93VXCB8Q5"
identifier "com.displaylink.DisplayLink-Manager-XPC-Client" and (
identifier "com.displaylink.DisplayLink-Manager-XPC-Client-ObjC" and (
identifier "com.displaylink.DisplayLinkUserAgent" and 
identifier "com.elgato.CameraHub" and certificate leaf[subject.OU] = "Y93VXCB8Q5"
info[CFBundleShortVersionString] >= "1.12.1"
info[CFBundleShortVersionString] >= "1.11.0"
[...]