Try to end the neverending fixIfBroken saga

Try to end the neverending fixIfBroken saga

This is a fixup to the  series of commits 522516d9, d0acc3e5, 13c3c148.

Actually the right thing to consider in all situations is isActive(),
which really means `can a cursor be in there?'.

Improve the description of isActive() and editable() in Inset.h.

Set isActive() to false for InsetInfo, since no cursor should ever go there.

Again part of #10667.