[gnome-books] (6 commits) Non-fast-forward update to branch wip/hadess/sensitive-menu

[Bastien Nocera <hadess src gnome org>]

The branch 'wip/hadess/sensitive-menu' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  c309c84... mainToolbar: Name a few of the headerbar items
  2c5ffc7... overview: Keep the main menu sensitive when no books
  3e9731f... overview: Make empty state icon a book
  0101ef7... overview: Make secondary label function extensible
  1deee62... overview: Add secondary label to empty state

Commits added to the branch:

  497fcd6... nav-bar: Fix possible crash when there's no pixbuf loader (*)
  53b5c78... mainToolbar: Name a few of the headerbar items
  b680f37... overview: Keep the main menu sensitive when no books
  600f356... overview: Make empty state icon a book
  b8fab35... overview: Make secondary label function extensible
  e40b0de... overview: Add secondary label to empty state

(*) This commit already existed in another branch; no separate mail sent