GUI: Allow engines to define a fully custom tab in the edit game dialog

By implementing MetaEngine::buildEngineOptionsWidget, engines can
instantiate a container widget that will be shown in the Engine tab of
the edit game dialog. The default implementation retains the existing
behavior and shows the extra GUI options.

gui/dialog.cpp

@@ -358,6 +358,10 @@ void Dialog::handleCommand(CommandSender *sender, uint32 cmd, uint32 data) {
 	case kCloseCmd:
 		close();
 		break;
+	case kCloseWithResultCmd:
+		setResult(data);
+		close();
+		break;
 	default:
 		break;
 	}