Pr/trails (#170)

* Implement new RenderableTrails as abstract base class
  - Implement RenderableTrailsOrbit and RenderableTrailsTrajectory as concrete instances
Remove old RenderableTrails and RenderableTrailsNew classes
Adapt mod files to the new structure

* Addressed Pull Request comments

src/engine/openspaceengine.cpp

@@ -895,13 +895,13 @@ void OpenSpaceEngine::postSynchronizationPreDraw() {
         _shutdownCountdown -= _windowWrapper->averageDeltaTime();
     }
 
+    _renderEngine->updateSceneGraph();
     _renderEngine->updateFade();
     _renderEngine->updateRenderer();
     _renderEngine->updateScreenSpaceRenderables();
     _renderEngine->updateShaderPrograms();
     
     if (!_isMaster) {
-        _renderEngine->updateSceneGraph();
         _renderEngine->camera()->invalidateCache();
     }