Don't clear away <hr>s RT#56465
Improve bugfix for RT#56465

- Define a list of usually empty, self closing block elements in html.js
- Add a set subtraction method to arrays.js
- use both in a improved fix in blockelementcontenthandler.js
Merge pull request #996 from wigentics/hotfix-rt56465

Fix for RT#56465

Some changes will need to be made to this patch after the merge...