Avoid sending any unecessary updates to the front-end and prevent hanging due to circular callbacks

[astrofrog]

This does two things:

• Fixes an infinite loop of callbacks in the layer options
• Avoids sending state updates to the front-end if the serialized JSON hasn't changed

@maartenbreddels - can you think of a better way to do the second of these? (the second commit in this PR)

[codecov]

Codecov Report

Base: 89.24% // Head: 89.24% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (6b91f6d) compared to base (2c1ccdd).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #325      +/-   ##
==========================================
- Coverage   89.24%   89.24%   -0.01%     
==========================================
  Files          86       86              
  Lines        4287     4296       +9     
==========================================
+ Hits         3826     3834       +8     
- Misses        461      462       +1     

Impacted Files	Coverage Δ
glue_jupyter/state_traitlets_helpers.py	86.27% <100.00%> (+2.94%)	⬆️
glue_jupyter/widgets/layer_options.py	94.44% <100.00%> (-5.56%)	⬇️

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[pllim]

Seems to not have broken Imviz but I also didn't try very hard.

Also seems like things are "snappier" now (e.g., the contrast/bias sliders) but I could be imagining it. I do not have numbers to back this up.