Epub Writer: Add cover reference to guide element

Fixes an issue with calibre http://calibre-ebook.com/ putting the
cover at the end of the book if the spine has linear="no".

Apparently this is best practice for other converters as well.

http://www.idpf.org/epub/20/spec/OPF_2.0.1_draft.htm#Section2.6
Epub Writer: Add cover reference to guide element (v2)

Avoiding an unnecessary list concatenation.

Fixes an issue with calibre http://calibre-ebook.com/ putting the
cover at the end of the book if the spine has linear="no".

Apparently this is best practice for other converters as well.

http://www.idpf.org/epub/20/spec/OPF_2.0.1_draft.htm#Section2.6
Merge pull request #1061 from heurist/epub_coverLinear

Epub Writer: Refactor unnecessary list ++