Add "machine" criterion to match WM_CLIENT_MACHINE

Closes #3981

Add "%machine" title_format placeholder
Add "machine" to the IPC and layout saving/restoring

parser-specs/commands.spec

@@ -54,6 +54,7 @@ state CRITERIA:
   ctype = 'title'       -> CRITERION
   ctype = 'urgent'      -> CRITERION
   ctype = 'workspace'   -> CRITERION
+  ctype = 'machine'     -> CRITERION
   ctype = 'tiling', 'floating'
       -> call cmd_criteria_add($ctype, NULL); CRITERIA
   ']' -> call cmd_criteria_match_windows(); INITIAL