Make command action scope robust

[Phrogz]

What: Fix the context visual-scxml-editor.editorActive to be active iff an editor is active, and switch commands that could be invoked from the text editor (like Add State) to use this context.
Why: Some commands make sense even if the visual editor is not active, but they cannot be invoked—and should not clutter the command palette—unless an SCXML text editor is active.