textview: Set background on the widget's window
textview: Set background on the widget's window

The widget window is usually covered by the bin_window.
Its background color will become relevant when we introduce
kinetic scrolling with overshooting.

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