fix(nexus): Add qRegisterMetaType call for ToxPk

Fixed warning message: QObject::connect: Cannot queue arguments of type
'ToxPk' (Make sure 'ToxPk' is registered using qRegisterMetaType().)

src/nexus.cpp

@@ -99,6 +99,7 @@ void Nexus::start()
     qRegisterMetaType<ToxFile>("ToxFile");
     qRegisterMetaType<ToxFile::FileDirection>("ToxFile::FileDirection");
     qRegisterMetaType<std::shared_ptr<VideoFrame>>("std::shared_ptr<VideoFrame>");
+    qRegisterMetaType<ToxPk>("ToxPk");
     qRegisterMetaType<ToxId>("ToxId");
 
     loginScreen = new LoginScreen();