Take disabled windows into account in wxFindWindowAtPoint() in wxMSW.

Use ChildWindowFromPointEx(CWP_SKIPINVISIBLE) to ensure that we find the
disabled children (by not using CWP_SKIPDISABLED).

Add a unit test to check for the correct behaviour in all cases and document
it.

Closes #2942.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70794 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/utils.h

@@ -404,6 +404,9 @@ void wxEnableTopLevelWindows(bool enable = true);
     Find the deepest window at the given mouse position in screen coordinates,
     returning the window if found, or @NULL if not.
 
+    This function takes child windows at the given position into account even
+    if they are disabled. The hidden children are however skipped by it.
+
     @header{wx/utils.h}
 */
 wxWindow* wxFindWindowAtPoint(const wxPoint& pt);