Screen blanking for presentations + additional hotkeys for navigation
Only support blankout in presentation mode

Also:
* no navigation in blankout mode
* leave blankout when presentation mode is left (e.g. by browser on ESC)
Support more API variants to enter/leave fullscreen (with input support)
Merge pull request #751 from kossebau/presentationBlanking

Presentation blanking (or: stealing #556)