[XML] Export functionality creates invalid XML

[andygrunwald]

Current Behavior

The export functionality at http://localhost:8080/index.php?mod=install&action=export&step=2 creates invalid HTML.
The functionality exports Database content in a raw fashion. If the content contains a & char (e.g. in a link), the XML output is invalid and can't be displayed in a browser.

Expected Behavior

XML export creates valid HTML.

Possible Solution

I think the XML class should be split into a XMLReader and XMLWriter.
This makes the responsibility clear.

The XMLReader can be supported via Symfony DomCrawler Component: https://symfony.com/doc/current/components/dom_crawler.html

The XMLWriter should use https://www.php.net/manual/en/book.xmlwriter.php to ensure proper XML is written.

Steps to Reproduce (for bugs)

1. Create a entry in the Beamer module that contains an & character
2. Go to http://localhost:8080/index.php?mod=install&action=export&step=2
3. Export the Beamer module
4. Download the XML and view it in a browser