Added option to remove duplicate entries from node selection

I've noticed that duplicate part entries can end up in the SCENARIO(ResearchAndDevelopment)/Tech nodes, which looks to slow down loading the tech tree.  Duplicates also contribute to visual clutter, and, I'd guess in many cases, without adding really any technical benefit.  This change adds a context menu option to remove duplicate "key = value" entries from a selection of nodes (and their descendants).