To allow for passing moderation errors around without impacting the State, this commit also separates the previous State enum into two enums: State, and Error. The State enum now only contains generic states like disconnected or connected, and the Error enum describes the specific error happened.
citra_qt/multiplayer/{state, message} is changed accordingly.
In dark theme, it is very hard to see the connected/disconnected icons because they are dark too. So I added two white-coloured icons for the dark theme. This also fixed an issue where theme update does not change the icon on the status bar.
