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 :)
From a quick play with LibreOffice, it seems that almost *any* key simply leaves blankout mode. I confirmed that all the following also exit blanked mode: left/right/up/down arrows, pageup/down, home/end, n/p, space.