(REF) AfformSaveTrait - Extract helper $isChanged

(REF) AfformSaveTrait - Extract helper $isChanged

I was originally expecting to need to flush the menu whenever the
`permission` changed.  That's not necessary now (because the true permission
check is deferred until it's needed), but the cleanup may still be helpful
in the future (e.g.  if we enable nav-menu integration or change the way
permissions are implemented).