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 :)
An alternative approach to this (not better, just alternate) would be to use classes instead. E.g., ``blankOut("white")`` which would add a class such as ``blanked.classList.add("white");``. Then you could add a CSS rule such as ``#blanked.white { background-color: white; display: block; }`` 

Only difference this provides really is that this approach would result in less hard-coded CSS in our JS files.