widget: don't call gtk_style_context_set_background if app_paintable
widget: don't call gtk_style_context_set_background if app_paintable

When a widget is app_paintable, its background should not be drawn by
the theme, thus we should not try to override its background again when
style-updated is fired.
This is a bit of a hack, but it fixes gray surfaces observed for DnD
windows with recent GTK+.

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