is a convenience interface for accessing several aspects of a form component
which supports validation.
A validatable form component has two aspects which other parties might
be interested in:
The pure validity flag: Whatever the user enters in the component, this is
either valid (relative to the active validator), or invalid.
The current value: Even if the validity flag does not change with the user
input (e.g. because the user replaces one invalid value with another invalid
value), observers might be interested in the current value, for example to
include it in a feedback message to the user.
