Add Plot Options and Subset Tools to default layout

[rosteen]

We don't really advertise the default layout at the moment, but I couldn't resist messing with it a bit while we discussed it offline, and realized that it needs the Plot Options plugin since we moved all the layer options and such into there. Had to handle the case where there are no viewers at all.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 73.33333% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 92.04%. Comparing base (f8b3191) to head (b5db6ac). :warning: Report is 1642 commits behind head on main.

Files with missing lines	Patch %	Lines
jdaviz/app.py	0.00%	4 Missing :warning:
jdaviz/core/template_mixin.py	83.33%	3 Missing :warning:
jdaviz/configs/default/helper.py	83.33%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1622      +/-   ##
==========================================
- Coverage   92.05%   92.04%   -0.02%     
==========================================
  Files         140      141       +1     
  Lines       15287    15309      +22     
==========================================
+ Hits        14073    14091      +18     
- Misses       1214     1218       +4     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[pllim]

There is a conflict now that needs to be resolved.

[rosteen]

Given that there's renewed interest (at GlueCon) in the default config/mode, it should be noted that this is going to need at least some conflict resolution with current main and likely a bit more work to bring it in line with some basic infrastructure work since last fall.