Layer data written with node.set {props: {pluginData}} was saved but no
command read it back. node.get (the command, the control method and MCP
get_node) and document.inspect now report pluginData as {key: value}
when a layer or page has any, so a tool keeping state in a file can
read it on its next run. Widget keys are included: the plugin rules
only stop plugins writing them, and widget.list already reports that
state.
