Backported fix for backward-compatibility issue on XMLSanitizer

[alongosz]

🎫 Issue	Follow-up to IBX-9336

Description:

This is a 3.3 backport of ibexa/fieldtype-richtext#225

This PR fixes breaking change detected during release process.

XMLSanitizer no longer performs XML decoding on the whole document. Instead, decoding is done only when relevant lines are being processed (for safe entity detection), and is otherwise left alone and unchanged.

This prevents decoded XML (which is invalid) from making it's way into the DOMDocumentFactory.

For QA:

Sanities & check failing regression build.