Refactor in Prefab system

[ncannasse]

Currently the prefab system is a bit hard to work with because of the factory model:

• some prefabs does have a member variable referencing their scene object, so they won't work in multiple references
• it is difficult to deal with the Context API, because you can't store all per-instances variables you might want
• serialization of fields is done by hand (load/save)
• modifying prefab data at runtime lead to this data being modified for later instances

I propose we change the following:

• each prefab have serialization through macros (@:s) + custom save/load if needed
• when doing make, we clone() the prefab then makeInstance the clone, so we keep the original "factory" prefab untouched but the instantiated one will hold a reference to the scene object (it be its own Context).
• remove Context (or actually ContextShared become Context)

Ping @trethaller @D0pu for comments

[ncannasse]

After this refactor, another refactor in edit():

• we want a quick way to edit prefab fields
• some metadata is required (float ranges)
• undo should be taken care directly
• we want to be able customize the UI

Instead of having a lot of metadata in the prefab vars decl that will not work when wanting to do custom things, the idea is instead to be able to declare things very briefly in edit():

For example:

group("Advanced", [
    range(alpha, 0, 1),
    bool(props.visible),
]); 
...

[ncannasse]

• remove Library (use directly Prefab)
• add loadDeps so references and binary data is loaded before make() -- refactor prefab loading using Library.loadFile that does loadData+loadDeps
• review prefab cache & reload