GDK W32: Fix the code to compile

GDK W32: Fix the code to compile

Somewhat change the order of initialization (to be closer
to what Wayland backend does).

Also remove the wrapper field that is no longer needed -
it used to hold a pointer to the main GdkWindow instance,
which wrapped GdkWin32ImplWindow. Since impls are gone,
nothing is wrapping anything anymore.

Fix a substitution error, where wrong pointer was added
to the hash table. Added a comment to ensure that future readers
(including myself) won't be confused by the fact that we're
inserting a pointer instead of the handle itself.