Open previews of different posts in different windows/tabs. Prevent confusion...

Open previews of different posts in different windows/tabs. Prevent confusion when using a former preview window again if the user navigated to edit another post. If the first post is still being edited, clicking Preview there will use the current window to show the preview. Props ippetkov, fixes #20233.
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git-svn-id: https://core.svn.wordpress.org/trunk@26952 1a063a9b-81f0-0310-95a4-ce76da25c4cd