* main: refactor MESSAGe_CREATE event to separate function
* main: add thread channel as child to guild text channel
* main: write thread channel messages to messages TextView on select
* *: refactor to use for range loops
* main: write discord.Message.ReferencedMessage if message type is ThreadStarterMessage
* fix(util): write Message.Content if it is not an empty string
* refactor(util): remove WriteMessages
* refactor(main): defer-close websocket connection
* refactor(main): update api.UserAgent to latest
* refactor: use named return values
* fix: increase guilds TreeView fixedSize
* docs(README): update preview
