UI extension exposes some non-API methods and classes

The UI extension code exposes some methods and classes that are not part of the public API. These should not be available to end users.

Steps to reproduce:

1. Build the UI extension library locally:
   cd ui-extension && mvn clean verify
2. Pack the extension:
   npm pack
3. Create another simple node module:
   cd <somewhere else>
   npm init -y
   npm i --save /path/to/ui-extension/bloomreach-ui-extension.0.0.0.tgz
4. Create a TypeScript file and edit it in IntelliJ
   touch index.ts
5. Add the following code:

import UiExtension from '@bloomreach/ui-extension';

UiExtension.register().then((ui) => {
  ui.
}); 

The TypeScript auto-complete now suggests the method "ui.init()", which is not part of the public API.

Also, it's possible to import several internal classes. For example:

import { Ui } from '@bloomreach/ui-extension';

These classes should not be exported.