gdk-win32: Really Implement GdkScreen->is_composited()

gdk-win32: Really Implement GdkScreen->is_composited()

The current GdkScreen->is_composited() is a stub as we were having Windows
XP being supported, which does not support Desktop Window Manager (DWM),
which is used by Windows for composition.

Windows Vista and later support DWM, and it is always enabled on Windows 8/
Server 2012 and later.

Please note that as we are dropping XP support in this cycle, this is the
commit that would say goodbye to Windows XP support for GTK+-3.x, by
linking directly to dwmapi.dll.  This means, we only check whether we are
on Windows 8 or Server 2012 (or later) to see whether we unconditionally
have composition enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=741849