internal links broken because of file extension changes to xhtml

 * Xreader version: xreader 2.6.4
 * Distribution: Linux Mint 20 Ulyana

Issue

After opening an epub, internal links are broken because the filename extensions on targets is changed from .html to .xhtml

Steps to reproduce

1. visit: http://www.gutenberg.org/ebooks/63748
2. download and save the "Epub with images"
3. open with xreader (as "Document Viewer")
4. click on LIST OF ILLUSTRATIONS
5. click on any illustration link. I chose the "Mary Boyle" link
6. You will get this reported error:
   Error opening file /tmp/xreader-125855/pg63748-images.epubY9LQT0/OEBPS/@public@vhost@g@gutenberg@html@files@63748@63748-h@63748-h-5.htm.html: No such file or directory
7. The reason for the error is that the file became 63748-h-5.htm.xhtml when it was burst into /tmp/xreader-125855

Expected behaviour

The links should work and does work with every other ebook reader I tried, such as fbreader and Calibre.

Other information

[clefebvre]

EPUB support was removed from Xreader and reimplemented in Xepub: https://github.com/xapp-project/xepub.