gtkplacesview: only filter out placeholder if searching
gtkplacesview: only filter out placeholder if searching

We were filtering out placeholders if the list box filters
while not searching, which is not what we want, since placeholders
should only be hidden if the view is searching.

https://bugzilla.gnome.org/show_bug.cgi?id=754150

chergert/gtk,jigpu/gtk,jadahl/gtk,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk