Refactor/project reorganization

[CSSFrancis]

This pull request introduces significant refactoring and code cleanup to both the Python and JavaScript codebases for Anyplotlib's plotting panels. The main changes include the introduction of shared base classes and mixins for plot panels in Python, and the consolidation of duplicated logic into shared helper functions in the JavaScript frontend, especially for view state preservation and pointer-settled event scheduling. These changes improve maintainability, reduce code duplication, and clarify responsibilities between components.

These changes collectively improve the maintainability, readability, and consistency of the Anyplotlib codebase.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.98477% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.84%. Comparing base (020fb92) to head (a7cc17d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
anyplotlib/conftest.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   84.38%   90.84%   +6.45%     
==========================================
  Files          29       31       +2     
  Lines        2645     2501     -144     
==========================================
+ Hits         2232     2272      +40     
+ Misses        413      229     -184     

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