GDK W32: Use a dumb window class for decorative windows

GDK W32: Use a dumb window class for decorative windows

Currently only one kind of decorative window is in use - the shape
indicator that is shown when snapping windows to the edge of the screen.

When normal toplevel class is used, its window procedure expects certain
motions from GDK (passing user data to CreateWindowEx(), registering
handle in a hash map etc), and might crash if that is not done.

Dumb window doesn't require anything, it can just be.

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