message: Use new delegate Button constructor in GameOptionsDialog
diff: GameOptionsDialog::GameOptionsDialog() :
  Window{constants::WINDOW_SYSTEM_TITLE},
  - btnSave{"Save current game"},
  - btnLoad{"Load a saved game"},
  - btnReturn{"Return to current game"},
  - btnClose{"Return to Main Menu"}
  + btnSave{"Save current game", {this, &GameOptionsDialog::onSave}},
  + btnLoad{"Load a saved game", {this, &GameOptionsDialog::onLoad}},
  + btnReturn{"Return to current game", {this, &GameOptionsDialog::onReturn}},
  + btnClose{"Return to Main Menu", {this, &GameOptionsDialog::onClose}}
  {
  position({0, 0});
  size({210, 160});
  add(btnSave, {5, 25});
  btnSave.size({200, 25});
  - btnSave.click().connect(this, &GameOptionsDialog::onSave);
  add(btnLoad, {5, 53});
  btnLoad.size({200, 25});
  - btnLoad.click().connect(this, &GameOptionsDialog::onLoad);
  add(btnReturn, {5, 91});
  btnReturn.size({200, 25});
  - btnReturn.click().connect(this, &GameOptionsDialog::onReturn);
  add(btnClose, {5, 129});
  btnClose.size({200, 25});
  - btnClose.click().connect(this, &GameOptionsDialog::onClose);
  anchored(true);
  }
  