Add function to remove unused media files

This is my first pull request at all. So please excuse if there are mistakes.

I put the function in openxml_document class, because it should be used by presentation and docx_part classes. The function gets executed during presentation -> remove_slide() and it should delete all media files, that aren't referenced in any xml.rels file.

Hello! Could you already have a look at this pull request? Anything I should fix/improve?

Hello! Could you already have a look at this pull request? Anything I should fix/improve?

@SteffenWolfFA-ERF : To accept a PR, it needs to pass all tests, some of which failed (see below). Do you want to try to fix them? You may run devtools::check() locally to identify most issues.

PS. Please be aware that you will also need to sync your fork and merge the changes that have happened in the master branch since May, when you submitted your PR.