Classification editor: use correct request type and template when adding new graph
Classification editor: speak of graphs instead of trees
Classification graphs: add option to get all graphs not linked to a project
Classification graphs: allow to link an existing graph to an existing project
Classification editor: offer to select a specific graph if more than one linked
Classifiation editor: make sure project and link IDs are of a numeric type
Classification editor: return the link id, too, if editor content is returned
Classification editor: refactor editor content handling and display a selected graph