Tools: Add bash completions for xkbcli

Tools: Add bash completions for xkbcli

- Add bash completion script. It parses the commands help messages to
  provide the completions, thus any new subcommand or option will be
  supported, as long as it has its entry in the help messages. This
  should result in low maintenancei effort.
- Add installation entry in Meson. The path can be configured using
  the following options:
  - `enable-bash-completion` to enable the installation;
  - `bash-completion-path` to control the installation path. It will
    default to: `share/bash-completion/completions`.

TODO: completion for other shells, such as zsh?