file chooser widget: Expose a "subtitle" property
file chooser widget: Expose a "subtitle" property

Give the file chooser widget a "subtitle" property, which
contains a short string that explains the current mode.

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