Add SingleLineEditor feature to mask passwords.

Add SingleLineEditor feature to mask passwords.

The implementor of an EditableMenuLineData trait can now set the trait
constant SECRET to indicate that the editor should display the text
as ******* while editing. However, the trait is responsible for doing
the same masking itself when displaying the data in non-editing
mode.

(This division of labour is necessary so that display() can also apply
other display features, such as complaining that you haven't given a
password at all yet, or that they don't match. display() is the only
thing that can know _which_ of its output needs masking.)