i3bar: Add protocol for workspace buttons

Closes #3818 (parent issue)
Fixes #1808
Fixes #2333
Fixes #2617
Fixes #3548

parser-specs/config.spec

@@ -528,6 +528,7 @@ state BAR:
   'set' -> BAR_IGNORE_LINE
   'i3bar_command'          -> BAR_BAR_COMMAND
   'status_command'         -> BAR_STATUS_COMMAND
+  'workspace_command'      -> BAR_WORKSPACE_COMMAND
   'socket_path'            -> BAR_SOCKET_PATH
   'mode'                   -> BAR_MODE
   'hidden_state'           -> BAR_HIDDEN_STATE
@@ -567,6 +568,10 @@ state BAR_STATUS_COMMAND:
   command = string
       -> call cfg_bar_status_command($command); BAR
 
+state BAR_WORKSPACE_COMMAND:
+  command = string
+      -> call cfg_bar_workspace_command($command); BAR
+
 state BAR_SOCKET_PATH:
   path = string
       -> call cfg_bar_socket_path($path); BAR