GtkPlacesSidebar: Cancel async ops when finalized
GtkPlacesSidebar: Cancel async ops when finalized

This is the common practice for objects that start async operations.
https://bugzilla.gnome.org/show_bug.cgi?id=735953

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