gdk: Select the X11 backend a the last possible backend
gdk: Select the X11 backend a the last possible backend

On Windows and OS X we want to prefer the native backends over the X11
backend.
On Linux, nothing changes as nobody is going to enable those backends
(and if they do, they'll know what they get).

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