GDK W32: custom (non-WM) drag-move and drag-resize code

GDK W32: custom (non-WM) drag-move and drag-resize code

Normally works only on CSD windows, non-CSD windows continue
to use WM modal loop for drag-resizing and drag-moving. However,
if it is activated on non-CSD windows, it does work.

Has the advantage of being completely immune to AeroSnap.
AeroSnap only worked partially on CSD windows, with the only part
that worked being "don't let users drag window titlebar outside of
the desktop". Now AeroSnap doesn't work on windows moved by
this code at all, which is good, since they currently don't work
well with it due to the way shadows are drawn.

It's possible to also re-implement AeroSnap (or something similar),
but that is a story for another commit.

This code was originally intended to fix the problem of window
size and window contents desynchronization, but failed to achieve
that result in the end. Nevertheless, it serves as a foundation for
other changes to the way window resizing works.

https://bugzilla.gnome.org/show_bug.cgi?id=761629