[Level Instances] Editor Actor Folders does a full refresh after any LI is loaded, leading to wasted time when bulk loading LIs for a given WP region load

From Licensee:

We discovered a pretty big performance hit in editor when loading level instances for a world partition map, specifically:

EditorActorFolders.cpp:

• FActorFolders::OnAllLevelsChanged()

• this method gets called any time an LI is loaded in the editor
• this method will do housekeeping and a full rebuild of the folder list for the world (expensive)
• This method will also get called multiple times for nested LIs, which adds to the cost of nested LIs impacting load times.

Proposed optimization:

• We added a global variable in a WITH_EDITOR block, for GIsBulkLoadingLevelInstances
• Within LevelInstanceSubsystem.cpp, add a TGuardValue<bool> to flip it on when doing bulk loading of LevelInstancesToLoadOrUpdateCopy
• After bulk loading is completed, broadcast an event & bind OnAllLevelsChanged to that event, so it can run the rebuild once.

Making this change on our end showed savings of between 40-60% for loading of level instances for out large/dense maps.