Cleanup touch/fullscreen handling.

[szimek]

• Use touch and fullscreen (on double tap/click) API on non-mobile page as well.
• Disable scroll only on mobile page.
• Move fullscreen API polyfill to the top outside of the game code.
• If fullscreen API works good enough it might not be necessary for address bar hack (there's no address bar in fullscreen mode in mobile Chrome and as home screen app on mobile Safari)
• Center board vertically in mobile view (in non fullscreen API version)