Modularize saving and loading into customizable phases

[muit]

Describe the feature requested
Split the process of saving and loading into modules or phases that can be customized:

• Saving & loading files
• Serializing process
  • Serializing world

How does it help the user (you)?
Adds a lot of flexibility for different games with different needs

Is it related to a problem?
A related problem is that some games change what they save or load during runtime.
One example is a game in which when the player dies he keeps its inventory but doesn't load world.

[Optional] Propose an approach to the feature (the less we think the better!)
Using module blueprints (or c++ objects) that build a sequence of saving orders.
OnSerialize() -> SerializePlayerInventory -> SerializeWorld -> If(playerIsDead) { SerializeAI() }

[Optional] Additional context
Original idea from issue #10

[muit]

Delayed a little bit to release 1.2 with 4.22

[muit]

The serialization module should allow to customize how everything is saved from blueprints.

This includes saying what to save. Map actors, custom picked actors etc.
Its in a way like building a pipeline by calling functions on blueprints.

[muit]

This feature is now part of Save Graphs(#39).
Closing for tracking purposes.