When users type, delete from, cut from, or paste to a StyledText,
four events fire:
key verification,
verification,
modification, and
extended modification.
Before the StyledText allows the changes to itself,
It first processes the key verification and the verification.
These handlers, VerifyKeyListeners and VerifyListeners, can allow, veto, or alter the requested text change.
After the change has happened, ModifyListeners and ExtendedModifyListeners react to the changes.