- Stubbed the undo implementation in TB2::revert (undo as a name is
already taken), and in the process lose the capability.
- Add detection of <modifications> when running the 'undo' command
and generate an error.
- Obsoleted Command::exectute 'commandLine' argument. It is worse
than unnecessary, it is an uncategorized raw argument string, which
is only really useful for the 'execute' command, which itself now
calls Arguments::combine to reconstruct the command line string.