GtkRecentManager: Survive without a filename
GtkRecentManager: Survive without a filename

We were asserting priv->filename != NULL in various places,
which leads to apps refusing to work when HOME is set to a
nonexisting or nonwritable value. Since it isn't hard, just
make GtkRecentManager survive without a filename. We won't
save or read any recently used files in this state, but
thats ok.
https://bugzilla.gnome.org/show_bug.cgi?id=739038

alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,Adamovskiy/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,chergert/gtk,Lyude/gtk-,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,ahodesuka/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,jadahl/gtk,Lyude/gtk-,chergert/gtk,Lyude/gtk-,Lyude/gtk-,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,Lyude/gtk-,alexlarsson/gtk,chergert/gtk,jigpu/gtk