message: Fix incorrect use of wxMenuItem::SetBitmap()
  Previously the method was called called after the item has been appended
  to the menu. wx docs say that SetBitmap() must be called before the item
  is appended to the menu.
  This broke the icons and caused Gtk-CRITICAL asserts under GTK+.
  See
diff: @@ -406,9 +406,10 @@ protected:
  // we need to quote '&' characters which are used for mnemonics
  menuEntry.Replace("&", "&&");
  - auto item = menu->Append(wxID_FILE1 + n, wxString::Format("&%d %s", n + 1, menuEntry));
  + auto item = new wxMenuItem(menu, wxID_FILE1 + n, wxString::Format("&%d %s", n + 1, menuEntry));
  item->SetHelp(fn.GetFullPath());
  item->SetBitmap(m_icons_cache->get_small(fn.GetExt()));
  + menu->Append(item);
  }
  file_icons_ptr m_icons_cache;
  