Extend _gdk_windowing_window_at_pointer to be able to get toplevels only

This has two advantages:
1) In many backends, this is faster as we can terminate the window
hierarchy traversal earlier
2) When used in gdkdisplay.c::get_current_toplevel() to get the
current toplevel that has the pointer we now correctly return
a toplevel with the pointer in it where the pointer is inside
some foreign subwindow of a toplevel window.

The second advantage fixes some bugs in client side event generation
when the pointer is inside such a foreign child window.

gdk/win32/gdkwindow-win32.c

@@ -2261,7 +2261,8 @@ GdkWindow*
 _gdk_windowing_window_at_pointer (GdkDisplay *display,
 				  gint       *win_x,
 				  gint       *win_y,
-				  GdkModifierType *mask)
+				  GdkModifierType *mask,
+				  gboolean    get_toplevel)
 {
   GdkWindow *window;
   POINT point, pointc;
@@ -2283,6 +2284,11 @@ _gdk_windowing_window_at_pointer (GdkDisplay *display,
   ScreenToClient (hwnd, &point);
 
   do {
+    if (get_toplevel &&
+	(window = gdk_win32_handle_table_lookup ((GdkNativeWindow) hwnd)) != NULL &&
+	GDK_WINDOW_TYPE (window) != GDK_WINDOW_FOREIGN)
+      break;
+
     hwndc = ChildWindowFromPoint (hwnd, point);
     ClientToScreen (hwnd, &point);
     ScreenToClient (hwndc, &point);