add the possibility to link one or more detached base layer to a normal base layer so more than one base layer can be show or hidden at once (this also works with the layer linked to the opacity slider)
Merge pull request #606 from camptocamp/linked_base_layers

add the possibility to link one or more detached base layer to a normal base layer so more than one base layer can be show or hidden at once (this also works with the layer linked to the opacity slider)