[node-view] Use 'hidden' as the attribute's value

Even though the attribute's effect gets activated for any value of the
attribute, this way it's easier for anyone looking at the DOM to figure
out what the visibility of the node is.
[node-view] Document node._isHidden
Merge branch 'node-hidden' into dev-master
Editor: IE 11 no longer supports `document.selection`, in favor of `window.getSelection`.
Editor: Build.