Introduce PasswordElement class

[yhabteab]

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[yhabteab]

@cla-bot check

[nilmerg]

Web 2's ResourceConfigForm doesn't include the stored password in the DOM. I think we should do the same but natively in the element itself. I believe this should work, since ipl\Html\Form can be (and in this case is) always being populated with the configuration:

• Don't use property $value of ipl\Html\FormElement\Password to render the value attribute, use a constant dummy password instead
  • Use an attribute callback for this
• Don't accept the dummy password as value for property $value (otherwise the real password is overridden and cannot be retrieved again by getValue())

This should ensure that:

• A call to setValue() with the real password still stores it in property $value (Done by FormElements::populate())
• A call to getValue() still returns the real password, if one got set by setValue() (Done by FormElements::getValues())
• The form doesn't render the real password in the DOM, but shows that a password is set
• The form accepts a new password

[nilmerg]

There's more, but I'd like to tackle that in person.