history transition should be able to save the graph to a file

From https://bugzilla.suse.com/show_bug.cgi?id=927475

> Transition usage:

```
transition [||] [nograph] [v...] [scores] [actions] [utilization]
transition showdot [||]
transition log [||]
transition save [|| [name]]
```

> Without a subcommand the local pengine is used to calculate and show the graph.
>
> Subcommand showdot displays the .dot file delivered with the report.
>
> graph usage (which shows the configuration of the PE):

`graph [ [ []]]`

> Perhaps sth like:

`transition graph [||] [ [ []]]`

> Or to extend showdot (graph is already used hence may cause confusion):

`transition showdot [||] [ []]`

> In the latter case we'd need to default to the local pengine to provide the graph if there's no dot file in the report.

Start with the CLI entry points for the transition and graph commands, especially transition showdot, transition save, and graph. Compare the proposed transition graph and extended showdot forms, including behavior when a report has no dot file. Done means the chosen command can save the requested history graph to a file with a defined image format and source behavior.

Written by the indexing model from the issue text.