support workspace_buttons protocol for swaybar

[quantenzitrone]

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• i3 PR: i3bar: Add protocol for workspace buttons i3/i3#4311

• Description:
  https://i3wm.org/docs/i3bar-workspace-protocol.html

The program defined by the workspace_command configuration option for i3bar can modify the workspace buttons displayed by i3bar. The command should constantly print in its standard output a stream of messages following the protocol defined in this page.

Sways implementation of this would need to be a bit different i guess, since the output of swaymsg -t get_workspaces --raw has a lot more information than i3s i3-msg -t get_workspaces.