Validators

[timdeschryver]

Currently validators are not re-invoked when a formfield becomes disabled/hidden.
This results in invalid form states that can't be corrected, or errors become invisible when a field is hidden.

Just like the Angular Forms API, the validators should be retriggered and become valid when that's the case.
To be able to still support disabled controls with validators, I propose a new "readOnly" property that has the same behavior as the current implementation of disabled.

This results in:

• disabled: retriggers validation, which is always valid because validators are ignored
• hidden: retriggers validation, which is always valid because validators are ignored
• readOnly: retriggers validation, validates the field with the current validators