generate unique names for nodes in 3d models

[eeropomell]

This PR introduces a dictionary in Model.cs for having a unique id for each node in a 3d model. It fixes string handling related issues in ModelWidget, #794

Todo:

• Measure Performance: when the dictionary is initialized, the entire transform hierarchy is traversed during model load.
• Currently it rewrites each node's name with the uid. This should probably be changed so the transform has the real name and instead of using Transform.Find() for node search, a custom method is implemented.
  • The user doesn't see the node names but it still makes debugging harder.

[mikeage]

Please make sure you're either using pre-commit locally or something else (Rider?) that handles the formatting as defined in .editorconfig

[eeropomell]

This PR is ready for review @andybak

TLDR:

• performance-wise, the implementation is good enough. On Meta Quest 3 it takes < 3ms to traverse a tree of 1000 nodes. Most users are importing models with < 1000 nodes. If this is to be optimized further, we should before that figure out if there are users who are using models that have > 100k or > 1 million nodes.

• the current implementation appends the node's UID to the node's name (e.g., "root uid: 1234567"), which is a simple and functional solution. We don't have to modify code in e.g ModelWidget.cs. A future improvement could focus on a cleaner implementation of unique identifiers without modifying the node names.

Profiling:

I didn't want there to be an insane frame freeze when the user imports a 3d model and the node tree is traversed and unique names are generated.

Some notes:

1. The profiling was done on Meta Quest 3, which has a Qualcomm Snapdragon XR2 CPU.
2. The performance of node tree traversal and unique name generation depends on CPU and RAM, not e.g., GPU. My current understanding is that other VR headsets OB is supported on have about equally powerful CPUs. For example, Vive Elite XR also has a Qualcomm Snapdragon XR2 CPU.
3. When profiling, the main thing that matters in the input is how many nodes the tree has. For example, how much geometry data the model has doesn't at least directly affect how fast we can traverse the node tree.

For a model with 1000 empty nodes, it takes ~3ms to traverse the tree and generate unique names:
Here is an image from Unity Profiler, showing the EndCreatePrefab marker taking 2.9ms:

This video shows importing models with 100, 1k, 10k, and 100k nodes. The model has completed loading and the unique names have been generated when the preview appears (which for these models is empty as there's no geometry data). Note that the node tree traversal is only a part of the import.

• With 100 and 1k, import is instant
• With 10k, import is about 0.5 seconds.
• With 100k nodes, the entire import takes a few seconds, and there's a ~1 sec frame freeze at the end, which is caused by the node tree traversal.

0376-0889.mp4