Issue/453 (#556)

* Introduced guiName to PropertyOwner
  * Added requirement that PropertyOwner::identifier may not contain whitespaces
  * Changed Name to Identifier in asset and scene files
  * Added new PropertyOwner to RenderEngine that owns the ScreenSpaceRenderables
  * Moved Name and GuiPath into GUI group
  * Added user-facing names to layer groups

include/openspace/rendering/loadingscreen.h

@@ -84,7 +84,8 @@ public:
         Failed
     };
 
-    void updateItem(const std::string& itemName, ItemStatus newStatus, float progress);
+    void updateItem(const std::string& itemIdentifier, const std::string& itemName,
+        ItemStatus newStatus, float progress);
 
 private:
     bool _showMessage;
@@ -122,6 +123,7 @@ private:
     std::mutex _messageMutex;
 
     struct Item {
+        std::string identifier;
         std::string name;
         ItemStatus status;
         float progress;