Remove gdk_window_add_filter

Remove gdk_window_add_filter

Drop the public filtering API. The x11 backend already has
the ::xevent signal as replacement. The win32 backend needs
a similar signal to replace filtering.

Reshuffle header inclusions in the x11 backend a little bit
to avoid a cyclic inclusion between gdkprivate-x11.h and
gdkdisplay-x11.h that is otherwise causing problems.