GDK W32: Refuse to release mouse grab while in DnD mode

GDK W32: Refuse to release mouse grab while in DnD mode

Handle WM_CANCELMODE and do nothing in response to it when DnD is
active. Otherwise pass it to DefWindowProc, which will call ReleaseCapture()
on our behalf.
This prevents us from losing mouse capture when alt-tabbing during DnD
(this includes the feature of Windows Explorer where dragging stuff over
a window button in the taskbar causes that window to receive focus, i.e.
keyboardless alt-tabbing).

https://bugzilla.gnome.org/show_bug.cgi?id=786509