XCOMMONS-497: Embedded SVG images are no longer displayed in the PDF export
* Still remaining an issue with title

XCOMMONS-496: Can't write empty DIV inside HTML macro

XCOMMONS-32: Extra new line in body content when cleaning HTML containing a XML declaration followed by a new line

Note 1: We've deployed a custom HTMLCleaner JAR ATM till the HTMLCleaner project releases an official 2.7 version (real soon now). We'll need to use that version before we can release XWiki Commons (since all our deps need to be on Maven Central!).
Note 2: If this fix works fine on master (5.4-milestone-1-SNAPSHOT) then we'll be able to backport it to 5.3 and even 5.2 depending on when HTMLCleaner 2.7 is released.