const CFStringRef kMIDIPropertyConnectionUniqueID;
device/entity/endpoint property, integer or CFDataRef
UniqueID of an external device/entity/endpoint attached to this one
(strongly recommended that it be an endpoint).
This is for the use of a setup editor UI; not currently used internally.
A driver-owned entity or endpoint has this property to refer
to an external MIDI device that is connected to it.
The property is non-existant or 0 if there is no connection.
New for CoreMIDI 1.1.
Beginning with CoreMIDI 1.3, this property may be a CFDataRef containing
an array of big-endian SInt32's, to allow specifying that a driver object connects
to multiple external objects (via MIDI thru-ing or splitting).
This property may also exist for external devices/entities/endpoints,
in which case it signifies a MIDI Thru connection to another external
device/entity/endpoint (again, strongly recommended that it be an edpoint).