Add functions for getting xid of a Window for unix and MacOS

For #133

window.cxx

@@ -75,11 +75,20 @@ void go_fltk_Window_size_range(Fl_Window* w, int minW, int minH, int maxW, int m
   w->size_range(minW, minH, maxW, maxH, deltaX, deltaY, aspectRatio);
 }
 
-#ifdef _WIN32
-void* go_fltk_Window_win32_xid(Fl_Window* w) {
+void *go_fltk_Window_xid(Fl_Window *w) {
+#if defined(_WIN32)
   return fl_win32_xid(w);
-}
+#elif defined(__APPLE__)
+  return fl_xid(w);
+#elif defined(__unix__)
+// TODO: Recognize in runtime if we're using Wayland or X11
+#if FLTK_USE_X11
+  return (void*)(uintptr_t)fl_xid(w);
+#elif FLTK_USE_WAYLAND
+  return fl_wl_xid(w);
+#endif
 #endif
+}
 
 const int go_FL_CURSOR_DEFAULT = (int)FL_CURSOR_DEFAULT;
 const int go_FL_CURSOR_ARROW = (int)FL_CURSOR_ARROW;

window.go

@@ -83,6 +83,10 @@ func (w *Window) SetSizeRange(minW, minH, maxW, maxH, deltaX, deltaY int, aspect
 	C.go_fltk_Window_size_range((*C.Fl_Window)(w.ptr()), C.int(minW), C.int(minH), C.int(maxW), C.int(maxH), C.int(deltaX), C.int(deltaY), C.int(ratio))
 }
 
+func (w *Window) RawHandle() uintptr {
+	return uintptr(C.go_fltk_Window_xid((*C.Fl_Window)(w.ptr())))
+}
+
 type Cursor int
 
 var (

window.h

@@ -24,9 +24,7 @@ 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 void* go_fltk_Window_xid(Fl_Window* w);
 
   extern const int go_FL_CURSOR_DEFAULT;
   extern const int go_FL_CURSOR_ARROW;