EZP-23708: As a developer, I want to have a basic XmlText fields support both in read and write mode

JIRA: https://jira.ez.no/browse/EZP-23708

# Description

The goal is to provide a basic support for XmlText in PlatformUI. *Basic* means that will display and edit XML code directly. (Note this PR is done against the branch of #125 to avoid having to change code inside the test files)

Screencast: https://www.youtube.com/watch?v=W1hNP8Gf-W4

## Tasks

* [x] Display the XmlText fields
* [x] Allow to edit the xml in a simple textarea

# Tests

manual + unit tests