always show viewer-level options regardless of selected layer

[kecnry]

Description

#2543 re-ordered entries in the plot options so that viewer-level options occur before the layer dropdown. This is confusing when the visibilities of some of these entries depend on the selected layer. This PR makes it so that the visibility of these viewer-level options is only based on the selected viewer(s) and not layer(s).

For example, in Imviz with multiselect enabled and NO layer selected, colormap was not showing at all before this PR, but now shows regardless of the selected layer(s).

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

All modified and coverable lines are covered by tests ✅

Files	Coverage Δ
...nfigs/default/plugins/plot_options/plot_options.py	91.20% <100.00%> (-0.03%)	⬇️
jdaviz/core/template_mixin.py	91.65% <100.00%> (+0.01%)	⬆️

📢 Thoughts on this report? Let us know!

[rosteen]

Seems to work, thanks.