Offer a way to hide bottom toolbar

Available in menu like sidebar and with <Ctrl>-F9 Shortcut (slightly following
GNOME Builder there)

Only for LibarayWindow. Fullscreen and Slideshow offer their own means of
(auto-)hiding the toolbar already.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=741962