Make limits cache private for serialization

[Carifio24]

This PR resolves #471 by using the first approach mentioned in that issue - changing limits_cache -> _limits_cache so that it isn't serialized. As mentioned in #471, this matches the structure of the VisPy viewer states.

I suppose this is a breaking change as it's changing the name of the class member. I would imagine that there aren't any clients directly manipulating the limits cache, but if there are I'm happy to use the second approach mentioned in the issue instead.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 86.03%. Comparing base (54499a8) to head (ee93916).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
glue_jupyter/common/state3d.py	57.14%	3 Missing ⚠️
glue_jupyter/common/tests/test_state3d.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #474      +/-   ##
==========================================
- Coverage   86.07%   86.03%   -0.05%     
==========================================
  Files          90       91       +1     
  Lines        5242     5269      +27     
==========================================
+ Hits         4512     4533      +21     
- Misses        730      736       +6     

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