gdk: Make window moves a custom vfunc

gdk: Make window moves a custom vfunc

The window move code needs special attention for multiple reasons:
- invalid areas for expose events need to be modified
- self-copy is not supported by Cairo
- in X11, copying from an overlapped Window might cause unexposed areas
  to be copied in, spo expose events for those need to be generated.

This was all special cased in various parts of the code. By making it an
explicit vfunc, we can work around it.