Refactor event polling code.

[gdamore]

This centralizes much of the logic (hopefully reducing duplication) for polling events and the queue. This will make it easier to make further design changes to express a better, simpler, API to consumers.

While here addressed missing logic to handle Fini correctly on Windows.

[codecov]

Codecov Report

Attention: 61 lines in your changes are missing coverage. Please review.

Comparison is base (e3a99dd) 45.58% compared to head (28c7846) 45.80%.

Files	Patch %	Lines
console_win.go	0.00%	22 Missing ⚠️
screen.go	45.45%	18 Missing ⚠️
tscreen.go	0.00%	15 Missing ⚠️
simulation.go	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #667      +/-   ##
==========================================
+ Coverage   45.58%   45.80%   +0.22%     
==========================================
  Files          79       79              
  Lines        8314     8280      -34     
==========================================
+ Hits         3790     3793       +3     
+ Misses       4495     4458      -37     
  Partials       29       29              

Flag	Coverage Δ
unittests	45.80% <25.60%> (+0.22%)	⬆️

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