Add Model::setLightPosition.

+ Convert some std::strings to QStrings.

src/app/qtkscene.cpp

@@ -485,13 +485,7 @@ void QtkScene::update()
   // Helper lambda to set the light position used by GLSL shaders on the model.
   // TODO: This could be a helper function on the Model class.
   auto setLightPosition = [](const std::string & lightName, Model * model) {
-    if (auto light = MeshRenderer::getInstance(lightName.c_str()); light) {
-      QVector3D position = light->getTransform().getTranslation();
-      model->setUniform("uLight.position", position);
-    } else {
-      qDebug() << "[QtkScene] Failed to set light position: "
-               << lightName.c_str();
-    }
+
   };
 
   QMatrix4x4 posMatrix;