Automatically persist objects between scene changes and on initialization.

This is meant to be used for things like preserving object positions between changing scenes, so they remain there when you return. Any attribute should be possible to persist, but it hasn't been comprehensively tested.

Usage:

• Add PersistComponent as a child of the object you want to persist and supply a unique_id.
• Set attributes to persist in the attributes array (e.g. position, rotation, scale, etc.)
• To save, call persist() on PersistComponent.
• To load, call load_data() on PersistComponent (also happens automatically if load_on_ready is true)