gtkdnd: restore drag window movement for the unmanaged case

gtkdnd: restore drag window movement for the unmanaged case

5bb12474d975ee4b790c5 removed the dnd window movement code to let
the gdk backends handle the window movement instead. While this
works for X11/wayland the win32 backend still uses the unmanaged
interface and expects the window movement to be handled on the gtk
side. This restores the functionality in case the dnd is unmanaged.

This fixes the drag window on Windows being stuck in the top left
corner instead of following the drag position.

https://bugzilla.gnome.org/show_bug.cgi?id=781737