Decouple GdkWindowCache life-cycle from GdkX11DragContext
Decouple GdkWindowCache life-cycle from GdkX11DragContext

By making window caches refcounted. This fixes problems with leaking
drag contexts, as experienced in
https://bugzilla.gnome.org/show_bug.cgi?id=637691
and
https://bugzilla.gnome.org/show_bug.cgi?id=144324

Based on a patch by drago01@gmail.com

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