obs. vs. obs for all grouped plots using `BasePlot`

[yugeji]

• [x] Closes #1876 and reimplements stale PR #2055 zhangguy:dotplot2d
• [ ] Empty group currently breaks
• [ ] dendrogram doesn't work with all plots
• [ ] I don't think anything would break if you just removed var_name reordering from the dendrogram entirely
• [ ] add tests using groupby_cols params
• [ ] Docs!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 54.23729% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 75.12%. Comparing base (6542113) to head (1c4740e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2769      +/-   ##
==========================================
- Coverage   75.24%   75.12%   -0.13%     
==========================================
  Files         116      116              
  Lines       12802    12847      +45     
==========================================
+ Hits         9633     9651      +18     
- Misses       3169     3196      +27     

Files	Coverage Δ
scanpy/plotting/_anndata.py	84.98% <100.00%> (ø)
scanpy/plotting/_docs.py	100.00% <ø> (ø)
scanpy/plotting/_tools/__init__.py	77.28% <ø> (ø)
scanpy/testing/_helpers/__init__.py	100.00% <100.00%> (ø)
scanpy/plotting/_matrixplot.py	95.69% <66.66%> (-1.01%)	:arrow_down:
scanpy/plotting/_dotplot.py	91.28% <71.42%> (-0.61%)	:arrow_down:
scanpy/plotting/_stacked_violin.py	81.48% <62.50%> (-3.02%)	:arrow_down:
scanpy/plotting/_baseplot_class.py	86.47% <34.78%> (-3.42%)	:arrow_down: