Presentation blanking (or: stealing #556)

To help getting #556 into master, I picked the latest commit from @leenaars master branch (https://github.com/leenaars/WebODF/commit/015dd0053dd8df444334580630f9ffb7025d028a , which seems to reflect more or less what that PR was about lately) and added another commit of mine with some fixes/changes I propose :)
AFAIK the idea of ViewerJS is to be used only in that simple way as promoted on its website, so using ids is okay for now. Thankfully the ViewerJS code is not that big, so switching all to using classes instead one day should not be a problem. At least using ids is consistent for now. :princess: