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 :)
Opacity is unnecessary.