let calculated attributes reference previous ones

[tommy-mor]

Alyssa requested this feature, and I think I've been able to make it work.

out.mp4

[codecov]

Codecov Report

Merging #2185 (9ae6b0c) into master (ad14432) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2185      +/-   ##
==========================================
- Coverage   88.03%   88.00%   -0.03%     
==========================================
  Files         247      247              
  Lines       22968    22968              
==========================================
- Hits        20219    20214       -5     
- Misses       2749     2754       +5     

Impacted Files	Coverage Δ
...gs/component_arithmetic/qt/component_arithmetic.py	82.14% <ø> (ø)
glue/viewers/common/state.py	84.21% <0.00%> (-10.53%)	⬇️
glue/conftest.py	59.09% <0.00%> (-3.04%)	⬇️
glue/core/layer_artist.py	80.00% <0.00%> (-0.58%)	⬇️

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