This is quite similar to PDF, except that by default the File menu
doesn't advertise this command. Possible to add it via Tools ->
Customize -> Menus -> Search -> EPUB.

(Reasoning for having a dedicated UNO command for EPUB is an increasing
amount of users look at EPUB as the new PDF, given how non-reflowable
PDF is hard to read on mobile devices, while EPUB is primarily
reflowable.)

Change-Id: Ib80f784a630a192f5b6ffbbfc60cb565fda7ed37