CSS: Add the 'visibility' property.

http://www.w3.org/TR/CSS21/visufx.html#visibility

'visibility: collapse' is not supported yet because tables are not, either.

This fixes #1329.
auto merge of #1331 : SimonSapin/servo/visibility, r=pcwalton

http://www.w3.org/TR/CSS21/visufx.html#visibility

`visibility: collapse` is not supported yet because tables are not, either.

This fixes #1329.