Support for multiple validation errors #314

[TysonMN]

Resolves #314

This PR adds support for multiple validation errors.

Here are some interesting points about this implementation.

• Other than adding to the API support for a list of errors, I left the existing API as is. Internally, I converted each Option and ValueOption to a list.
• Equality of errors is done by the equality for string list. This means that the order of errors matters. I think that is reasonable.
• The sample still includes the previous code in which there is only one validation error. I added another text box that includes multiple validation errors.
• To quickly get this PR working, the Submit message in the sample no longer takes in the parsed data. However, doing so is preferred. As Mark Seemann recently said, validation is a solve problem via applicative functors. Unfortunately, F# doesn't include a Validation<,> monad. I have created one in my project at work. I could add that to the sample but that would also increase the learning curve. Alternatively, one could just parse the input under the assumption it will succeed (via int) because the canExec function returned true. This is not a good practice to do everywhere though.
• The red box around the text box containing the password is too long on the right side. I don't know why and didn't try to fix it either.

[cmeeren]

The CI build fails, could you investigate?

[TysonMN]

Ah, thanks for pointing that out. Will do.

[TysonMN]

CI build passes now. I forgot to update the tests to match the new behavior. I force pushed the needed changes.

[cmeeren]

I pushed a few commits. What do you think?

The rectangle width error was because the error message was wider than the input field. I increased the width of the input field.

[TysonMN]

Great improvements. Looks good to me.

Ready to merge?

[cmeeren]

Yep, thanks a lot!