Escape labels in UmlDotGraphStyle

[nightroman]

UmlDotGraph does not escape special characters like " and \ in state,
trigger, description labels. As a result, the generated DOT is invalid when
such labels exist.

This is not a big deal perhaps when labels are carefully selected in order to
work around this issue. But state machines may be dynamically created from
data/strings not originally designed with Stateless and this issue in mind.

This PR includes:

• new method UmlDotGraphStyle.EscapeLabel
• UmlDotGraphStyle uses this method when required
• added the new test SimpleTransitionWithEscaping, similar to SimpleTransition but using problematic labels
• removed the test SimpleTransitionUML because it seems to be completely identical to SimpleTransition
• reworked the existing test SpacedUmlWithSubstate so that it uses problematic states, triggers, descriptions

P.S. I hit this issue with real use cases on making state machines in PowerShell for using with FarNet.Stateless.

[nightroman]

Any feedback please? If it's not being addressed, I'd rather close the PR than have it in my TODO list...

[crozone]

Hi @nightroman, just getting around to reviewing this, thanks for the bump. It might take a day or two.