Add textual mouseover descriptions of collapsed nodes

[hyanwong]

I realised that the numbers can be somewhat cryptic, and it's trivial to add helpful text that appears on mouseover, e.g. "This polytomy has 3 additional branches, leading to a total of 14 descendant samples"

PR Checklist:

• Tests that fully cover new/changed functionality.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (9dcae00) to head (df5eb80).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3051   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files          29       29           
  Lines       32150    32155    +5     
  Branches     5768     5768           
=======================================
+ Hits        28890    28895    +5     
  Misses       1859     1859           
  Partials     1401     1401           

Flag	Coverage Δ
c-tests	86.71% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	89.05% <ø> (ø)
python-tests	98.98% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
python/tskit/drawing.py	99.24% <100.00%> (+<0.01%)	⬆️