Adds command_name as a custom command that can be used by this.
You must call add_command before send_message in order to use the newly added command.
The command name is used internally: you need to use the command's logical id in
send_message and inside the UniqueApp:UniqueApp:message-received signal.
| this |
a App |
| command_name |
command name |
| command_id |
command logical id |