Fixed Execute->Repeat Previous menu item.

[ci skip] - there's no need to run the travis build for a UI change.
Added scene output to ExecutableRender nodes.

This means they can be seen in the Viewer, SceneHierarchy and SceneInspector.
Added pass-through plugs for ImageWriter and SceneWriter.

This allows them to be viewed in the UI.
Removed unnecessary serialisation of pass-through plugs.
Merge pull request #991 from johnhaddon/repeatLastDispatch

Fixed Execute->Repeat Previous menu item.