Why not simply use PM's Structured Keywords Panel? It is really easy to add keywords, synonyms and categories, particularly now that these are separately colour coded and in different fonts. You cannot drag and drop, but you can cut and paste to a different location as a child or sibling.
If you do not want to do that, you can use a spreadsheet, such as Excel, and save your list in tab delimited format. If you need to see how this works, simply import an exported keyword list into Excel. Maybe it is easier in Excel if you are making wholesale, major moves of keywords between higher level categories, but for myself I find the Structured Keywords Panel more than good enough. No need, in my view, for yet another specialist application.
Hope that helps.
Another Graham