Add function for getting raw handle of a window under Windows

For #133

window.cxx

@@ -2,6 +2,7 @@
 
 #include <FL/Fl_Window.H>
 #include <FL/Fl_Double_Window.H>
+#include <FL/platform.H>
 
 #include "event_handler.h"
 
@@ -71,9 +72,15 @@ void go_fltk_Window_set_icons(Fl_Window* w, const Fl_RGB_Image *images[], int le
 }
 
 void go_fltk_Window_size_range(Fl_Window* w, int minW, int minH, int maxW, int maxH, int deltaX, int deltaY, int aspectRatio) {
-	w->size_range(minW, minH, maxW, maxH, deltaX, deltaY, aspectRatio);
+  w->size_range(minW, minH, maxW, maxH, deltaX, deltaY, aspectRatio);
 }
 
+#ifdef _WIN32
+void* go_fltk_Window_win32_xid(Fl_Window* w) {
+  return fl_win32_xid(w);
+}
+#endif
+
 const int go_FL_CURSOR_DEFAULT = (int)FL_CURSOR_DEFAULT;
 const int go_FL_CURSOR_ARROW = (int)FL_CURSOR_ARROW;
 const int go_FL_CURSOR_CROSS = (int)FL_CURSOR_CROSS;

window.h

@@ -24,6 +24,9 @@ extern "C" {
   extern void go_fltk_Window_set_non_modal(Fl_Window *w);
   extern void go_fltk_Window_set_icons(Fl_Window* w, const Fl_RGB_Image* images[], int length);
   extern void go_fltk_Window_size_range(Fl_Window* w, int minW, int minH, int maxW, int maxH, int deltaX, int deltaY, int aspectRatio);
+#ifdef _WIN32
+  extern void* go_fltk_Window_win32_xid(Fl_Window* w);
+#endif
 
   extern const int go_FL_CURSOR_DEFAULT;
   extern const int go_FL_CURSOR_ARROW;

window_windows.go

@@ -0,0 +1,12 @@
+//go:build windows
+
+package fltk
+
+/*
+#include "window.h"
+*/
+import "C"
+
+func (w *Window) RawHandle() uintptr {
+	return uintptr(C.go_fltk_Window_win32_xid((*C.Fl_Window)(w.ptr())))
+}