artnet dmx + leds outputs breaks discovery

[SpComb]

Having both dmx-output and leds artnet-enabled re-uses the same enum artnet_port (0..3) numbers for the artnet outputs, and thus only one of the outputs is included in the Art-Net PollReply.

The event_group bits used for the output events should be separate from the index, and the index should be allocated dynamically based on many outputs there are with the same artnet_port number.