Re: [fullscreen] Styles of former fullscreen elements (#18)

Reverting to use fullscreen element stack instead of top layer with fullscreen flag to manager the elements. In addition, add a sentence somewhere stays that when the head of the fullscreen element stack changes, remove the previous head from the top layer and add the new head into it.

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https://github.com/whatwg/fullscreen/issues/18#issuecomment-134495628