Fixup the fixup d0acc3e57044: use editable()/isActive()

Fixup the fixup d0acc3e57044: use editable()/isActive()

While 522516d9 was too strong and broke mathed, d0acc3e57044 is too
lenient and can accept insets (mathed/CommandInset, InsetInfo) that
have a positive nargs() but are not editable (because they encapsulate
something).

Therefore the best solution for now is to use editable() in text and
isActive() in mathed, until those two things are merged.

Part of #10667.