Add support for eBooks

Add a new application, "Books" (aka gnome-books) to the
gnome-documents git repository. This application shares most
of its code with Documents, and it supports:
- Categorising e-Books vs. Comics
- Searches
- Preview of CBZ/CBR/etc. comic formats (supported through the
  libevince backend) with "fit page" zoom by default

https://bugzilla.gnome.org/show_bug.cgi?id=704316