Artnet (#440)

* Started implementing artnet/DMX support

It compiles, but certainly wont work just yet

* Fix up packet data and length
correct default udp port

The data looks ok in wireshark

* Code cleanup
Sequence runs from 1..255 not 0
fix universe > 255

* code cleanups and force even number of channels

* Fix potential endianness issue

* added support for 'x' channels per fixture with zero padding
its very basic support for now - it needs better multi universe support

libsrc/leddevice/schemas/schema-artnet.json

@@ -0,0 +1,42 @@
+{
+	"type":"object",
+	"required":true,
+	"properties":{
+		"host" : {
+			"type": "string",
+			"title":"edt_dev_spec_targetIp_title",
+			"propertyOrder" : 1
+		},
+		"port" : {
+			"type": "integer",
+			"title":"edt_dev_spec_port_title",
+			"default": 6454,
+			"minimum" : 0,
+			"maximum" : 65535,
+			"propertyOrder" : 2
+		},
+		"universe": {
+			"type": "integer",
+			"title":"edt_dev_spec_universe_title",
+			"default": 1,
+			"propertyOrder" : 3
+		},
+		"channelsPerFixture": {
+			"type": "integer",
+			"title":"edt_dev_spec_chanperfixture_title",
+			"default": 3,
+			"propertyOrder" : 4
+		},
+		"latchTime": {
+			"type": "integer",
+			"title":"edt_dev_spec_latchtime_title",
+			"default": 1,
+			"append" : "edt_append_ms",
+			"minimum": 1,
+			"maximum": 1000,
+			"access" : "expert",
+			"propertyOrder" : 5
+		}
+	},
+	"additionalProperties": true
+}