Modified legend handling + Bug fixing

2 modifications with this update:

- ‘Literal value labels’ is now a third independent label that can be
turned on / off. Literal value nodes can no longer be hidden

- The full-length label of a literal value is fixed to represent the
complete XDI statement (including ‘/‘) that defines the corresponding
literal node. The fix applies to both loaded graphs or while adding a
literal value to an existing graph.