feat(WebSocket): add open event (#6315)

src/common/websockets/WebSocketPool.hpp

@@ -42,6 +42,11 @@ private:
 struct WebSocketListener {
     virtual ~WebSocketListener() = default;
 
+    /// The WebSocket handshake completed successfully.
+    ///
+    /// This function is called from the websocket thread.
+    virtual void onOpen() = 0;
+
     /// A text message was received.
     ///
     /// This function is called from the websocket thread.

src/common/websockets/detail/WebSocketConnectionImpl.cpp

@@ -194,6 +194,7 @@ void WebSocketConnectionHelper<Derived, Inner>::onWsHandshake(
 
     qCDebug(chatterinoWebsocket) << *this << "WS handshake done";
 
+    this->listener->onOpen();
     this->trySend();
     this->stream.async_read(
         this->readBuffer,