Let's think through what classes we need to implement with our new ValidatedMixin class, as follows:
- All our fields except Notes are required, so we'll need a basic Entry widget that registers an error if there's no input.
- We have one Date field, so we need an Entry widget that enforces a valid date string.
- We have a number of the Spinbox widgets for decimal or integer input. We'll need to make sure these only accept valid number strings.
- We have a few Combobox widgets that don't behave quite the way we want them to.
Let's get started!
