_clear_cache method in all template objects

[kecnry]

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Description

This pull request copies the _clear_cache method that exists for plugins to the TemplateMixin base class so plugin subcomponents and toolbar tools, etc, can use cached properties in a consistent way to what is currently used in (some) plugins.

Change log entry

• [x] 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.

• [x] 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?
• [x] 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[bot]]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.76%. Comparing base (581da08) to head (d68f47c). Report is 962 commits behind head on main.

Files with missing lines	Patch %	Lines
jdaviz/core/template_mixin.py	16.66%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2253      +/-   ##
==========================================
- Coverage   91.79%   91.76%   -0.03%     
==========================================
  Files         150      150              
  Lines       16785    16791       +6     
==========================================
+ Hits        15407    15408       +1     
- Misses       1378     1383       +5     

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