Compile and runtime fixes when running in Debug

Do not crash if ScreenSpaceLocalImage does not exist
Add ability to specify identifier/name when adding screenspace items in the UI

modules/webgui/webguimodule.cpp

@@ -149,7 +149,7 @@ void WebGuiModule::removeEndpointChangeCallback(CallbackHandle handle) {
     );
 
     ghoul_assert(
-        it != _deltaTimeChangeCallbacks.end(),
+        it != _endpointChangeCallbacks.end(),
         "handle must be a valid callback handle"
     );