Jsoneditor

- Fixed 566: transform function broken, regression since `v5.20.0`.

- Implemented support for custom validation using a new `onValidate` callback.
- In tree mode, nodes containing a validation error now have a className
`jsoneditor-validation-error` which can be used for custom styling.

- Implemented `onEvent` callback triggered when an event occurs in a JSON
field or value. Thanks cristinabarrantes.

- Show validation errors inline instead of at the bottom when in code
mode. Thanks meirotstein.
- Fix 562: allow `$` character in property names of of a JSON schema.

_Good news: JSONEditor is finally framework friendly and can now be easily
integrated in React, Vue, and Angular!_

- Implemented new methods `update` and `updateText`, which maintain the state
of the editor (expanded nodes, search, selection). This makes it easy to
integrate in frameworks like React.
- Implemented options `onChangeJSON(json)` and `onChangeText(jsonString)`.
- Added two React examples to the `examples` folder.
- Fixed menu buttons "Sort" and "Transform" being available in modes `view`
and `form`.

- Fixed 558: scrolling to search results and automatically scrolling up/down
when dragging an item broken (regression since v5.19.1).