Move closeDialog signal from Loading to LoginStep
This prevents errors because we try to connect to a non-existent signal if the step is anything but Loading.
Prevents this runtime error:
qrc:/org/kde/neochat/qml/WelcomePage.qml:92:13: QML Connections: Detected function "onCloseDialog" in Connections element. This is probably intended to be a signal handler but no signal of the target matches the name.