Make wxDumpWindow() public and available in wxMSW too

This function was defined in wxGTK and wxOSX, but not in wxMSW or the
other ports, but it can be useful there too, so make it public and
define it in common code.

interface/wx/window.h

@@ -4251,5 +4251,22 @@ wxWindow* wxGetActiveWindow();
 */
 wxWindow* wxGetTopLevelParent(wxWindow* window);
 
+/**
+    Return a string with human-readable platform-specific description of
+    the window useful for diagnostic purposes.
+
+    The string returned from this function doesn't have any fixed form and can
+    vary between different wxWidgets ports and versions, but contains some
+    useful description of the window and uniquely identifies it. This can be
+    useful to include in debug or tracing messages.
+
+    @param window Window pointer which is allowed to be @NULL.
+
+    @header{wx/window.h}
+
+    @since 3.1.6
+ */
+wxString wxDumpWindow(wxWindow* window);
+
 //@}