xC/xP: finalize and implement Event.PING

xP/public/xP.js

@@ -85,7 +85,8 @@ class RelayRpc extends EventTarget {
 			if (typeof e.event !== 'string')
 				throw "Invalid event tag"
 
-			this.dispatchEvent(new CustomEvent(e.event, {detail: e}))
+			this.dispatchEvent(new CustomEvent(
+				e.event, {detail: {eventSeq: message.eventSeq, ...e}}))
 
 			// Minor abstraction layering violation.
 			m.redraw()
@@ -153,6 +154,10 @@ rpc.addEventListener('close', event => {
 	m.redraw()
 })
 
+rpc.addEventListener('Ping', event => {
+	rpc.send({command: 'PingResponse', eventSeq: event.detail.eventSeq})
+})
+
 rpc.addEventListener('BufferUpdate', event => {
 	let e = event.detail, b = buffers.get(e.bufferName)
 	if (b === undefined) {