Stop using deprecated GTK_STOCK_* in GTK3.

According to the GTK3 docs, we're now supposed to use fixed label
strings instead.

unix/gtkcompat.h

@@ -1,7 +1,7 @@
 /*
  * Header file to make compatibility with older GTK versions less
  * painful, by #defining various things that are pure spelling changes
- * between GTK1 and GTK2.
+ * between GTK1 and GTK2, or between 2 and 3.
  */
 
 #if !GTK_CHECK_VERSION(2,0,0)
@@ -156,3 +156,12 @@
 
 #endif
 
+#if GTK_CHECK_VERSION(3,0,0)
+#define STANDARD_OK_LABEL "_OK"
+#define STANDARD_OPEN_LABEL "_Open"
+#define STANDARD_CANCEL_LABEL "_Cancel"
+#else
+#define STANDARD_OK_LABEL GTK_STOCK_OK
+#define STANDARD_OPEN_LABEL GTK_STOCK_OPEN
+#define STANDARD_CANCEL_LABEL GTK_STOCK_CANCEL
+#endif

unix/gtkdlg.c

@@ -1674,8 +1674,8 @@ static void filefont_clicked(GtkButton *button, gpointer data)
              (uc->ctrl->fileselect.for_writing ?
               GTK_FILE_CHOOSER_ACTION_SAVE :
               GTK_FILE_CHOOSER_ACTION_OPEN),
-             GTK_STOCK_CANCEL, GTK_RESPONSE_CANCEL,
-             GTK_STOCK_OPEN, GTK_RESPONSE_ACCEPT,
+             STANDARD_CANCEL_LABEL, GTK_RESPONSE_CANCEL,
+             STANDARD_OPEN_LABEL, GTK_RESPONSE_ACCEPT,
              (const gchar *)NULL);
 	gtk_window_set_modal(GTK_WINDOW(filechoose), TRUE);
 	g_object_set_data(G_OBJECT(filechoose), "user-data", (gpointer)uc);

unix/gtkfont.c

@@ -2996,9 +2996,9 @@ unifontsel *unifontsel_new(const char *wintitle)
     fs->u.window = GTK_WINDOW(gtk_dialog_new());
     gtk_window_set_title(fs->u.window, wintitle);
     fs->u.cancel_button = gtk_dialog_add_button
-	(GTK_DIALOG(fs->u.window), GTK_STOCK_CANCEL, GTK_RESPONSE_CANCEL);
+	(GTK_DIALOG(fs->u.window), STANDARD_CANCEL_LABEL, GTK_RESPONSE_CANCEL);
     fs->u.ok_button = gtk_dialog_add_button
-	(GTK_DIALOG(fs->u.window), GTK_STOCK_OK, GTK_RESPONSE_OK);
+	(GTK_DIALOG(fs->u.window), STANDARD_OK_LABEL, GTK_RESPONSE_OK);
     gtk_widget_grab_default(fs->u.ok_button);
 
     /*