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 :)
Hm, might make sense to not limit that only to blanking keys, to catch all the cases where people want to do some op next. That way the only have to search one key and just double press it, instead of hunting down two different keys (reminds me that touch ui variant will also need some support one day).