GGU-PLGW: "Save nodes" menu item

After going to this menu item defined hydrograph points can be saved in order to have them available again at a later date. For reasons of clarity, it makes sense to use ".knt" as file suffix, as this is the suffix used in the file requester box for the menu item "Special/Load nodes". If you choose not to enter an extension when saving, ".knt" will be used automatically.