Introduce new WebSockets receiver impl

When enabled, this new receiver will spin up a local WebSockets server
and will send the currently playing song information to any clients that
connect. It's designed with Übersicht in mind, since WebSockets is the
easiest way to efficiently push events into an Übersicht widget, but
I'm sure it'd work for a variety of other purposes too.

Currently the socket is only used in one direction, pushing the current
song info from server to client, but I'll probably extend it to support
sending MPD commands from WebSockets clients as well.

schemata/config-v1.json

@@ -42,6 +42,46 @@
 			},
 			"title": "MpdConfig",
 			"type": "object"
+		},
+		"WebsocketsReceiverConfig": {
+			"properties": {
+				"host": {
+					"anyOf": [
+						{
+							"format": "hostname",
+							"type": "string"
+						},
+						{
+							"items": {
+								"format": "hostname",
+								"type": "string"
+							},
+							"type": "array"
+						}
+					],
+					"title": "Host"
+				},
+				"kind": {
+					"const": "websockets",
+					"default": "websockets",
+					"enum": [
+						"websockets"
+					],
+					"title": "Kind",
+					"type": "string"
+				},
+				"port": {
+					"maximum": 65535,
+					"minimum": 1,
+					"title": "Port",
+					"type": "integer"
+				}
+			},
+			"required": [
+				"port"
+			],
+			"title": "WebsocketsReceiverConfig",
+			"type": "object"
 		}
 	},
 	"$id": "https://cdn.00dani.me/m/schemata/mpd-now-playable/config-v1.json",
@@ -65,13 +105,17 @@
 			"items": {
 				"discriminator": {
 					"mapping": {
-						"cocoa": "#/$defs/CocoaReceiverConfig"
+						"cocoa": "#/$defs/CocoaReceiverConfig",
+						"websockets": "#/$defs/WebsocketsReceiverConfig"
 					},
 					"propertyName": "kind"
 				},
 				"oneOf": [
 					{
 						"$ref": "#/$defs/CocoaReceiverConfig"
+					},
+					{
+						"$ref": "#/$defs/WebsocketsReceiverConfig"
 					}
 				]
 			},