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 :)
Only partially related to this PR, but is the expectation to ever support multiple viewers in the same page without an iframe? If so, this should be a class-based lookup rather than an id-based one (e.g., "webodf-blanked").