`[sg]et-variable-by-index` considered confusing

There've been multiple instances of new FREDers trying to use `get-variable-by-index` and `set-variable-by-index` instead of Replace Variable, and being extremely confused by variables as a result. These operators are extremely niche, and now that we have containers, I'd even go so far as to call them obsolete. I think these operators should be hidden in FRED to avoid confusion.

Variable types also probably play into the confusion: creating a variable but getting the type wrong causes Replace Variable to be greyed out, which isn't good for discoverability; there might be room for a discussion about how to tweak this facet of the UI.