GdkWin32: Fix mouse events in presence of transparent windows on the desktop

GdkWin32: Fix mouse events in presence of transparent windows on the desktop

In gdkdevice-win32.c we are interested in knowing which window
receives mouse input at a specific location.

Only WindowFromPoint is the right API for the task, other API's
(such as (Real)ChildWindowFromPoint(Ex)) have shortcomings because
they are really designed for other purposes. For example, only
WindowFromPoint is able to look through transparent layered windows.

So even if we want to find a direct child we have to use
WindowFromPoint and then walk up the hierarchy.

Fixes: #370, #417
See: !2800