searchbar: Ignore 'space' & 'enter' when not focused

Doing so allows box selection to be toggled through 'enter' & 'space'
keys in selection mode. This makes use more accessible (as in a11y) and
this behavior is consistent with other GNOME apps (e.g Documents) and is
documented here:

https://help.gnome.org/users/gnome-help/stable/keyboard-nav.html.en

https://bugzilla.gnome.org/show_bug.cgi?id=706623