Entity Relations

[DasLixou]

Alternative to #188
Closes #187

This makes all Entity fields in blender selectable to objects, which translate to gltf nodes, so mesh selecting currently isn't possible yet.

On the bevy side, it uses a fake Entity overwrite with custom deserialization, which parses the name and over thread_local statics searches for the real entity id and replaces itself with that.

A previous attempt was to change the dyn Reflect tree, which currently isn't possible (bevyengine/bevy#14850) but could clean up a lot of stuff in the future.

One downside this approach currently has, is that I can only check whether an entity is a gltf node or mesh, which can share the same names, with GltfExtra and GltfMeshExtra, so when you want to select an object, it must at least have one component, otherwise it won't be able to find it.

On the blender side, it also fixes some code with values not having defaults, which with entities is now possible, and sets the default for boolean from True to False.

I haven't tested it yet, but this should also make joints from avian possible to declare in blender.

Testing

I did some basic things including the example, but external more extensive tests appreciated!
Having a reference in a collection which is instantiated multiple times also works correctly.

[DasLixou]

Ok, much of the unsafe code is gone now, should be much easier to read :D
(also, for spawning many many blueprints over a span of multiple frames might have a little perf improvement because query states are now cached)