container.Panel: add the ability to dynamically add headers

[tobiu]

With the latest changes, we can now use a panel as a container, in case no headers are defined initially.

In case someone would want to add headers into the instance at a later point, we need to enhance the logic.

Please add a comment, in case you need this!

At first, we should move the headers creation logic out of createItems() into a new method.

headers => headers_

afterSetHeaders() => check for new entries (not contained inside oldValue) and add them.

Follow up ticket: verticalHeadersFirst => verticalHeadersFirst_. The afterSet method would need to adjust the inner containers as needed.