win32: get rid of GdkDragContextPrivateWin32 and related machinery.
win32: get rid of GdkDragContextPrivateWin32 and related machinery.

This turns GdkWin32DragContext into a proper GdkDragContext subclass.
Because we now correctly initialize GdkWin32DragContext in
gdk_drag_context_new, we no longer crash immediatly when a DnD
operation is initialized (the find_window, drag_status, ... vfuncs
where all pointing to 0x0 instead of their proper win32 implementations).

We now try to consistently refer to GdkDragContext as "context",
GdkWin32DragContext as "win32_context" and the ole2 related
target_drag_context and source_drag_context as "ctx".

Members of GdkWin32DragContext only used by the ole2 DnD codepaths
are now explicitly marked with a ole2_dnd_ prefix.

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