Validation

[cassiozen]

- Summary

Adds validation to widgets. Available validations:

• Presence: By default all widgets are required, unless specified in the config. Example:
  - {label: "Subtitle", name: "subtitle", widget: "string", required: false}

• Pattern: Field configuration can specify a regex pattern with the appropriate error message. Example:
  - {label: "Title", name: "title", widget: "string", pattern: [".{10,}", "Should have more than 10 characters"] }

The widget control can optionally implement an isValid method to perform custom validations, in addition to presence and pattern. The isValid method will be automatically called, and it can return either a boolean value, an object with an error message or a promise. Examples:

Boolean
No errors:

  isValid = () => {
    // Do internal validation
    return true;
  };

Existing error:

  isValid = () => {
    // Do internal validation
    return false;
  };

Object with error (Useful for returning custom error messages)
Existing error:

  isValid = () => {
    // Do internal validation
    return { error: 'Your error message.' };
  };

Promise
You can also return a promise from isValid. While the promise is pending, the widget will be marked as in error. When the promise resolves, the error is automatically cleared.

  isValid = () => {
    return this.existingPromise;
  };

Note: Do not create a promise inside isValid - isValid is called right before trying to persist. This means that even if a previous promise was already resolved, when the user hits 'save', isValid will be called again - if it returns a new Promise it will be immediately marked as in error until the new promise resolves.

- Test plan

- Description for the changelog

Added support for validations.

- A picture of a cute animal (not mandatory but encouraged)

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[calavera]

Looking good so far