add loaded signal to entity backend and delay entity override loading

[nkrisc]

Description

Draft PR to fix array contents not being deserialized during loading when an entity contains override values for an array property.

This PR adds a signal to PandoraEntityBackend to indicate when loading is completed. On PandoraEntity, loading of overrides is delayed until that signal is emitted. This is because the root cause of the issue is that when the overrides are loaded, the properties have not yet been fully loaded and thus it's not (currently) possible to check the type setting of the array.

There are some other ways this could be solved though:

• Storing the array type with the serialized entity overrides
• Deserializing properties first and threading settings through the loading process to where the overrides are loaded

This issue would also likely affect any future types that contain other types, such as Dictionary.

The unit test is currently still failing, though in practice the issue seems at least initially resolved, but there may be other affected cases yet.

Addressed issues

• Array properties do not properly deserialize all types #159

TODO

• Ensure importing works and is unaffected

[nkrisc]

Not how I meant to close it, but I will open a new PR with a complete different approach to this issue, that I also think could set a precedent for a dictionary type as well.