ValidatedEntry: Mark entry as required

## Problem

Sometimes in a form there are entries which are required and ones which are optional, but at the moment it's not clear which are which

## Proposal

A property to mark a validated entry as being required. Perhaps this would add a status icon when the entry is empty.

## Prior Art

It seems like a common convention to use an asterisk (*) to denote required fields. This is usually pink or red.

However, since most entries are usually required in forms it may be better to mark entries which are optional instead. It's not clear that there is a conventional icon for this. It looks like maybe a plus, circle, gear, or "…", but it would probably not be a good idea to use something we use elsewhere as an action icon