Use threads for baking navigation mesh inside editor

[smix8]

Enables threaded navigation mesh baking inside the editor.

Closes #90120

The baking with threads inside the editor was disabled because especially the SceneTree caused crashes in the past.

Now that the problematic parsing step and the baking step are split and a few new guards exist the baking can run on a thread again.

Note that there are still editor freezes from the parsing process with this as that needs to stay on the main-thread due to the SceneTree and nodes.

Longer freezes are primarily caused by the RenderingServer when visual meshes are parsed because the RenderingServer needs to shuffle all that Mesh data from the GPU back to the CPU.

Use collision shapes as source geometry instead of visual meshes to avoid most issues!

[fire]

I am not sure of the implications but the feature is useful.

[akien-mga]

Thanks!