Fix mnemonic activation for some printer option widgets
Fix mnemonic activation for some printer option widgets

The file type radio group has a mnemonic on its label, but
activating it did not work, since GtkPrinterOptionWidget did
not know to forward the activation to one of the buttons.

msteinert/gtk,bratsche/gtk-,grubersjoe/adwaita,Lyude/gtk-,davidgumberg/gtk,jigpu/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Sidnioulz/SandboxGtk,chergert/gtk,davidt/gtk,msteinert/gtk,jigpu/gtk,jessevdk/gtk,msteinert/gtk,jadahl/gtk,Distrotech/gtk2,Distrotech/gtk2,Distrotech/gtk2,Distrotech/gtk2,bratsche/gtk-,alexlarsson/gtk,Adamovskiy/gtk,ahodesuka/gtk,grubersjoe/adwaita,davidgumberg/gtk,ahodesuka/gtk,bratsche/gtk-,alexlarsson/gtk,alexlarsson/gtk,ahodesuka/gtk,jigpu/gtk,bratsche/gtk-,msteinert/gtk,msteinert/gtk,jessevdk/gtk,Lyude/gtk-,jigpu/gtk,Lyude/gtk-,alexlarsson/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,ebassi/gtk,bratsche/gtk-,jessevdk/gtk,davidt/gtk,chergert/gtk,davidt/gtk,jadahl/gtk,ahodesuka/gtk,jessevdk/gtk,Adamovskiy/gtk,Lyude/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,Lyude/gtk-,davidgumberg/gtk,ebassi/gtk,davidt/gtk,Lyude/gtk-,Adamovskiy/gtk,Distrotech/gtk2,Distrotech/gtk2,jessevdk/gtk,Sidnioulz/SandboxGtk,chergert/gtk,ebassi/gtk,ebassi/gtk,ahodesuka/gtk,davidgumberg/gtk,chergert/gtk,Lyude/gtk-,jigpu/gtk,jessevdk/gtk,grubersjoe/adwaita,ahodesuka/gtk,chergert/gtk,grubersjoe/adwaita,ebassi/gtk,davidt/gtk,davidgumberg/gtk,jadahl/gtk,Adamovskiy/gtk,grubersjoe/adwaita,ahodesuka/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,Lyude/gtk-,davidt/gtk,Adamovskiy/gtk,msteinert/gtk,davidgumberg/gtk,jessevdk/gtk,ebassi/gtk,davidgumberg/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk