GDK W32: Add/subtract shadow when (un)snapping

GDK W32: Add/subtract shadow when (un)snapping

Now halfleft/halfright/fullup snaps do hug screen edges as intended.

Documents AeroSnap behaviour when snapped windows are drag-resized
(currently this implementation handles this in a very simplistic way).

Don't believe GTK when it tells us that window shadow is 0, preserve
previous values (but do remember that GTK wants no shadow, in case
we need that).

Fixes a couple of bugs in unsnapping (check offset against the half
of the window; don't put pointer in the middle of the window vertically
if it still fits in the top half).

https://bugzilla.gnome.org/show_bug.cgi?id=763013