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E-book Viewer: Use wiktionary.org and wordnik.com as dictionary providers instead of dict.org (which no longer exists).

When looking up words, the word definition is now opened in a separate
browser window.  The language of the dictionary to lookup is
automatically selected based on the language of the current book. Fixes #1349548 [dict.org not available](https://bugs.launchpad.net/calibre/+bug/1349548)