Refactor: hide implementation of GOOptionMenu (move from .h -> .c)

borrowed/goffice/go-optionmenu.c

@@ -50,6 +50,15 @@ enum
 
 static guint signals[LAST_SIGNAL] = { 0 };
 
+struct _GOOptionMenu
+{
+    GtkButton button;
+
+    GtkMenuShell *menu;
+    GtkMenuItem *selected;
+    GtkLabel *button_label;
+};
+
 G_DEFINE_TYPE (GOOptionMenu, go_option_menu, GTK_TYPE_BUTTON)
 
 GtkWidget*

borrowed/goffice/go-optionmenu.h

@@ -40,17 +40,6 @@ G_BEGIN_DECLS
 #define GO_TYPE_OPTION_MENU              (go_option_menu_get_type ())
 G_DECLARE_FINAL_TYPE (GOOptionMenu, go_option_menu, GO, OPTION_MENU, GtkButton)
 
-struct _GOOptionMenu
-{
-    GtkButton button;
-
-    /*< private >*/
-
-    GtkMenuShell *menu;
-    GtkMenuItem *selected;
-    GtkLabel *button_label;
-};
-
 GtkWidget* go_option_menu_new(void);
 void go_option_menu_set_menu(GOOptionMenu *option_menu, GtkWidget *menu);
 void go_option_menu_set_history(GOOptionMenu *option_menu, GSList *selection);