Prevent double call to open_file from double-clicking a book.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1030 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Set imagehandler's file list early to avoid threading problems.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1031 ef8490bc-f317-4725-81ed-e95a9aca1bcf
Close extractor after creating thumbnail, freeing the extraction thread.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1032 ef8490bc-f317-4725-81ed-e95a9aca1bcf