refactor!: add top level commands and new sub commands

[ludviglundgren]

This PR introduced a lot of breaking changes.

• setup top level subcommands and children
• update docs
• update flags and args on many commands
• new commands
• improved importers

Usage:
  qbt [command]

Available Commands:
  app         app subcommand
  bencode     bencode subcommand
  category    category subcommand
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  tag         tag subcommand
  torrent     torrent subcommand
  version     Print the version

Flags:
      --config string   config file (default is $HOME/.config/qbt/.qbt.toml)
  -h, --help            help for qbt

Use "qbt [command] --help" for more information about a command.