multiselect support for dropdown default

[kecnry]

Description

This pull request fixes the defaulting behavior of a plugin select component with multiselect enabled. Previously, even if the previous selection was valid, the current selection would be overwritten. Now, the previous selection is maintained. To reproduce: in imviz, create a second viewer, open plot options, enable multiselect, make a selection for the viewer, create or remove a viewer.

This type of workflow is more evident in #2341, so that should be rebased on top of this.

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)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• 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?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• 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.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Patch coverage: 82.60% and project coverage change: +0.02% 🎉

Comparison is base (eed8420) 90.63% compared to head (fbee0e0) 90.66%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2344      +/-   ##
==========================================
+ Coverage   90.63%   90.66%   +0.02%     
==========================================
  Files         152      152              
  Lines       17407    17439      +32     
==========================================
+ Hits        15777    15811      +34     
+ Misses       1630     1628       -2     

Files Changed	Coverage Δ
...configs/default/plugins/export_plot/export_plot.py	59.44% <69.56%> (+5.70%)	⬆️
jdaviz/core/template_mixin.py	91.18% <90.00%> (-0.01%)	⬇️
...configs/cubeviz/plugins/tests/test_export_plots.py	77.77% <100.00%> (+2.02%)	⬆️
...lt/plugins/plot_options/tests/test_plot_options.py	100.00% <100.00%> (ø)
jdaviz/configs/imviz/tests/test_viewers.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[rosteen]

I confirmed that the workflow you described behaves as expected, code looks fine. Approved.

[javerbukh]

Looks good!

[kecnry]

@meeseeksdev backport to v3.6.x