GGU-TIME-GRAPH: "Save as" menu item

You can save data entered during program use to an existing file or to a new file, i.e. using a new file name. For reasons of clarity, it makes sense to use ".gis" as file suffix, as this is the suffix used in the file requester box for the "File/Load" or "File/Append" menu items. If you choose not to enter an extension when saving, ".gis" will be used automatically.