Revert "Fix compilation warning: Use G_POINTER_TO_UINT"

Revert "Fix compilation warning: Use G_POINTER_TO_UINT"

GPOINTER_TO_UINT() *does* drop the upper 32 bits on 64-bit
Windows. After all, it by definition produces a (32-bit) unsigned int.

The GLib commit mentioned in the reverted commit's log message just
means that it does this without any compiler warning about casting
from pointer to integer of smaller size, as it first casts to a 64-bit
integer, then casts that to a (32-bit) int.

We must use the GdkNativeWindow cast here so that on 64-bit Windows
the gpointer data in embedded_list is converted to a (64-bit) native
window handle without going through 32-bit truncation. If ifdefs are
required to avoid compiler warnings on X11, so be it then. Probably it
would be best to introduce a GDK_GPOINTER_TO_NATIVE_WINDOW() macro in
gdktypes.h.

This reverts commit 78151dc7d41069e876fe165527e68c238ef7fd4a.