improve laggy subset finalization

[kecnry]

Description

This pull request limits the updates to subset_selected_changed_callback (originally introduced in #2939) to only trigger for subset update messages corresponding to the currently selected subset (and only when the subset state has been changed, not color, etc) and also minimizes spectral extraction calls when subset_select.items are changed to only affect the selected entry.

Fixes #3164

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

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• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
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• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.37%. Comparing base (69e31b7) to head (7b6b95d).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3175      +/-   ##
==========================================
- Coverage   88.42%   88.37%   -0.05%     
==========================================
  Files         122      122              
  Lines       18267    18278      +11     
==========================================
+ Hits        16153    16154       +1     
- Misses       2114     2124      +10     

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[kecnry]

FYI: @bmorris3 - does this seem reasonable? I'm trying to avoid an update to the state again if no new child layers are added.

[javerbukh]

I think I notice a marginal improvement on my windows PC and the code looks reasonable so I will approve. Looking forward to more optimization in this direction!

[rosteen]

I found this to be a very minor improvement, but this apparently isn't the source of most of the (millions of) calls to startswith that I was seeing. I think we can get it in, but let's not close the issue that started it (I'll have to remake that issue, it got deleted).

[rosteen]

Approved, but please don't close until @pllim has remade the deleted corresponding issue.

[kecnry]

I won't have this close #3181 since it doesn't fully address the lag (although it should address the unnecessary calls to spectral extraction during the subset creation process).