Group checkboxes doesn't update legend

[mapcentia]

          Maybe this needs an issue by it's own, but as I understand this is a related issue.

If you use the groupe button to enable all layers, the legend will not update with the layers enabled.

Only if you enable individual layers in the group the legend will update.

Originally posted by @hblGP in #286 (comment)

[giovanniborella]

this was fixed in #336