[WIP] 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

<img src="https://raw.githubusercontent.com/patrickallaert/patrickallaert.github.io/master/assets/travaux4.gif" alt="Building...">

The goal is to provide a basic support for XmlText in PlatformUI. *Basic* means that will display and edit XML code.

## Tasks

* [x] Display the XmlText fields
* [ ] Allow to edit the xml in a simple textarea