fix: Ensure necessary scripts are loaded into page

[mcollovati]

Description

In Liferay "Content Page" edit mode, Vaadin portlets are not directly
added to the main page, but loaded into an iframe requesting the same
page in preview mode, and then added to the main page through javascript,
but they are detached from iframe before initialization is completed.
This way, scripts needed to register the portlet web component are not
added to the main document, so the webcomponent is not correctly rendered.
This patch add an additional script to BootstrapHandler response that
takes care to add needed script to main page and to postpone element
registration until they are loaded.

Fixes #202

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[vaadin-bot]

SonarQube analysis reported 5 issues

1. PortletBootstrapHandler.java#L119: Either log or rethrow this exception.
2. VaadinLiferayPortlet.java#L49: Take the required action to fix the issue indicated by this comment.
3. VaadinPortlet.java#L641: A "NullPointerException" could be thrown; "portlet" is nullable here.
4. VaadinPortlet.java#L643: Define and throw a dedicated exception instead of using a generic one.
5. VaadinPortlet.java#L667: Remove these unused method parameters.

[mshabarov]

Tested with Liferay 7.3, works as expected.