[css-viewport] Move Visual Viewport (and more) from cssom-view to css-viewport

[bramus]

As it stands right now, the Visual Viewport is defined in the cssom-view spec. This feels like it’s in the wrong place, given that we have a css-viewport spec nowadays.

I think that that section, and by the looks of it the section on events too, should move to css-viewport. Some of the extensions to the Window interface should move too.

/ping editors @frivoal @emilio @smfr
/fyi @fantasai