#37 Unfix selected nodes in Layout

[Hugo Giesteira]

In the present version it is possible to select multiple nodes and select "unfix node". Despite the multiple selection the unfix is just applied to a single node. This "unfix node" option should be applied to all the selected nodes despite the node being a currency, reaction or metabolite.
Also the unfix should be applied to all the "twin" nodes of the selected node.